diff --git a/src/compiler.ts b/src/compiler.ts
index a1654463..94d17b02 100644
--- a/src/compiler.ts
+++ b/src/compiler.ts
@@ -1954,7 +1954,7 @@ export class Compiler extends DiagnosticEmitter {
       condExpr = module.createI32(1);
       alwaysTrue = true;
     }
-    innerFlow.inheritNonnullIf(condExpr);
+    innerFlow.inheritNonnullIfTrue(condExpr);
     var incrExpr = statement.incrementor
       ? this.compileExpression(<Expression>statement.incrementor, Type.void, ConversionKind.IMPLICIT, WrapMode.NONE)
       : 0;
@@ -2041,7 +2041,7 @@ export class Compiler extends DiagnosticEmitter {
     // Each arm initiates a branch
     var ifTrueFlow = outerFlow.fork();
     this.currentFlow = ifTrueFlow;
-    ifTrueFlow.inheritNonnullIf(condExpr);
+    ifTrueFlow.inheritNonnullIfTrue(condExpr);
     var ifTrueExpr = this.compileStatement(ifTrue);
     ifTrueFlow.freeScopedLocals();
     this.currentFlow = outerFlow;
@@ -2050,7 +2050,7 @@ export class Compiler extends DiagnosticEmitter {
     if (ifFalse) {
       let ifFalseFlow = outerFlow.fork();
       this.currentFlow = ifFalseFlow;
-      ifFalseFlow.inheritNonnullIfNot(condExpr);
+      ifFalseFlow.inheritNonnullIfFalse(condExpr);
       ifFalseExpr = this.compileStatement(ifFalse);
       ifFalseFlow.freeScopedLocals();
       this.currentFlow = outerFlow;
@@ -2058,7 +2058,7 @@ export class Compiler extends DiagnosticEmitter {
     } else {
       outerFlow.inheritConditional(ifTrueFlow);
       if (ifTrueFlow.isAny(FlowFlags.ANY_TERMINATING)) {
-        outerFlow.inheritNonnullIfNot(condExpr);
+        outerFlow.inheritNonnullIfFalse(condExpr);
       }
     }
     return module.createIf(condExpr, ifTrueExpr, ifFalseExpr);
@@ -2436,7 +2436,7 @@ export class Compiler extends DiagnosticEmitter {
     var continueLabel = "continue|" + label;
     innerFlow.continueLabel = continueLabel;
 
-    innerFlow.inheritNonnullIf(condExpr);
+    innerFlow.inheritNonnullIfTrue(condExpr);
     var body = this.compileStatement(statement.statement);
     var alwaysTrue = false; // TODO
     var terminated = innerFlow.isAny(FlowFlags.ANY_TERMINATING);
@@ -4759,7 +4759,7 @@ export class Compiler extends DiagnosticEmitter {
         let previousFlow = this.currentFlow;
         let rightFlow = previousFlow.fork();
         this.currentFlow = rightFlow;
-        rightFlow.inheritNonnullIf(leftExpr);
+        rightFlow.inheritNonnullIfTrue(leftExpr);
         rightExpr = this.compileExpression(right, leftType, ConversionKind.IMPLICIT, WrapMode.NONE);
         rightType = leftType;
         this.currentFlow = previousFlow;
@@ -4809,7 +4809,7 @@ export class Compiler extends DiagnosticEmitter {
         let previousFlow = this.currentFlow;
         let rightFlow = previousFlow.fork();
         this.currentFlow = rightFlow;
-        rightFlow.inheritNonnullIfNot(leftExpr);
+        rightFlow.inheritNonnullIfFalse(leftExpr);
         rightExpr = this.compileExpression(right, leftType, ConversionKind.IMPLICIT, WrapMode.NONE);
         rightType = leftType;
         this.currentFlow = previousFlow;
@@ -5030,7 +5030,6 @@ export class Compiler extends DiagnosticEmitter {
     var module = this.module;
     var flow = this.currentFlow;
     var target = this.resolver.resolveExpression(expression, flow); // reports
-    var possiblyNull = this.currentType.is(TypeFlags.NULLABLE) && !flow.isNonnull(valueExpr);
     if (!target) return module.createUnreachable();
 
     switch (target.kind) {
@@ -5043,7 +5042,7 @@ export class Compiler extends DiagnosticEmitter {
           this.currentType = tee ? (<Local>target).type : Type.void;
           return module.createUnreachable();
         }
-        return this.makeLocalAssignment(<Local>target, valueExpr, tee, possiblyNull);
+        return this.makeLocalAssignment(<Local>target, valueExpr, tee, !flow.isNonnull(this.currentType, valueExpr));
       }
       case ElementKind.GLOBAL: {
         if (!this.compileGlobal(<Global>target)) return module.createUnreachable();
@@ -5242,7 +5241,10 @@ export class Compiler extends DiagnosticEmitter {
       if (!flow.canOverflow(valueExpr, type)) flow.setLocalFlag(localIndex, LocalFlags.WRAPPED);
       else flow.unsetLocalFlag(localIndex, LocalFlags.WRAPPED);
     }
-    if (possiblyNull) flow.unsetLocalFlag(localIndex, LocalFlags.NONNULL);
+    if (type.is(TypeFlags.NULLABLE)) {
+      if (possiblyNull) flow.unsetLocalFlag(localIndex, LocalFlags.NONNULL);
+      else flow.setLocalFlag(localIndex, LocalFlags.NONNULL);
+    }
     if (tee) {
       this.currentType = type;
       return this.module.createTeeLocal(localIndex, valueExpr);
diff --git a/src/flow.ts b/src/flow.ts
index 8c090752..4a7227d1 100644
--- a/src/flow.ts
+++ b/src/flow.ts
@@ -582,8 +582,12 @@ export class Flow {
     this.localFlags = combinedFlags;
   }
 
-  /** Checks if an expression is known to be non-null. */
-  isNonnull(expr: ExpressionRef): bool {
+  /** Checks if an expression of the specified type is known to be non-null, even if the type might be nullable. */
+  isNonnull(type: Type, expr: ExpressionRef): bool {
+    if (!type.is(TypeFlags.NULLABLE)) return true;
+    // below, only teeLocal/getLocal are relevant because these are the only expressions that
+    // depend on a dynamic nullable state (flag = LocalFlags.NONNULL), while everything else
+    // has already been handled by the nullable type check above.
     switch (getExpressionId(expr)) {
       case ExpressionId.SetLocal: {
         if (!isTeeLocal(expr)) break;
@@ -598,8 +602,9 @@ export class Flow {
     return false;
   }
 
-  /** Sets local states where this branch is only taken when `expr` is true-ish. */
-  inheritNonnullIf(expr: ExpressionRef): void {
+  /** Updates local states to reflect that this branch is only taken when `expr` is true-ish. */
+  inheritNonnullIfTrue(expr: ExpressionRef): void {
+    // A: `expr` is true-ish -> Q: how did that happen?
     switch (getExpressionId(expr)) {
       case ExpressionId.SetLocal: {
         if (!isTeeLocal(expr)) break;
@@ -607,7 +612,7 @@ export class Flow {
         this.setLocalFlag(local.index, LocalFlags.NONNULL);
         break;
       }
-      case ExpressionId.GetLocal: { // local must be true-ish/non-null
+      case ExpressionId.GetLocal: {
         let local = this.parentFunction.localsByIndex[getGetLocalIndex(expr)];
         this.setLocalFlag(local.index, LocalFlags.NONNULL);
         break;
@@ -615,22 +620,24 @@ export class Flow {
       case ExpressionId.If: {
         let ifFalse = getIfFalse(expr);
         if (!ifFalse) break;
-        if (getExpressionId(ifFalse) == ExpressionId.Const && getExpressionType(ifFalse) == NativeType.I32 && getConstValueI32(ifFalse) == 0) {
+        if (getExpressionId(ifFalse) == ExpressionId.Const) {
           // Logical AND: (if (condition ifTrue 0))
-          // the only way this can become true is if condition and ifTrue are true
-          this.inheritNonnullIf(getIfCondition(expr));
-          this.inheritNonnullIf(getIfTrue(expr));
+          // the only way this had become true is if condition and ifTrue are true
+          if (
+            (getExpressionType(ifFalse) == NativeType.I32 && getConstValueI32(ifFalse) == 0) ||
+            (getExpressionType(ifFalse) == NativeType.I64 && getConstValueI64Low(ifFalse) == 0 && getConstValueI64High(ifFalse) == 0)
+          ) {
+            this.inheritNonnullIfTrue(getIfCondition(expr));
+            this.inheritNonnullIfTrue(getIfTrue(expr));
+          }
         }
         break;
       }
       case ExpressionId.Unary: {
         switch (getUnaryOp(expr)) {
-          case UnaryOp.EqzI32: {
-            this.inheritNonnullIfNot(getUnaryValue(expr)); // !expr
-            break;
-          }
+          case UnaryOp.EqzI32:
           case UnaryOp.EqzI64: {
-            this.inheritNonnullIfNot(getUnaryValue(expr)); // !expr
+            this.inheritNonnullIfFalse(getUnaryValue(expr)); // !value -> value must have been false
             break;
           }
         }
@@ -642,9 +649,9 @@ export class Flow {
             let left = getBinaryLeft(expr);
             let right = getBinaryRight(expr);
             if (getExpressionId(left) == ExpressionId.Const && getConstValueI32(left) != 0) {
-              this.inheritNonnullIf(right); // TRUE == right
+              this.inheritNonnullIfTrue(right); // TRUE == right -> right must have been true
             } else if (getExpressionId(right) == ExpressionId.Const && getConstValueI32(right) != 0) {
-              this.inheritNonnullIf(left); // left == TRUE
+              this.inheritNonnullIfTrue(left); // left == TRUE -> left must have been true
             }
             break;
           }
@@ -652,9 +659,9 @@ export class Flow {
             let left = getBinaryLeft(expr);
             let right = getBinaryRight(expr);
             if (getExpressionId(left) == ExpressionId.Const && (getConstValueI64Low(left) != 0 || getConstValueI64High(left) != 0)) {
-              this.inheritNonnullIf(right); // TRUE == right
+              this.inheritNonnullIfTrue(right); // TRUE == right -> right must have been true
             } else if (getExpressionId(right) == ExpressionId.Const && (getConstValueI64Low(right) != 0 && getConstValueI64High(right) != 0)) {
-              this.inheritNonnullIf(left); // left == TRUE
+              this.inheritNonnullIfTrue(left); // left == TRUE -> left must have been true
             }
             break;
           }
@@ -662,9 +669,9 @@ export class Flow {
             let left = getBinaryLeft(expr);
             let right = getBinaryRight(expr);
             if (getExpressionId(left) == ExpressionId.Const && getConstValueI32(left) == 0) {
-              this.inheritNonnullIf(right); // FALSE != right
+              this.inheritNonnullIfTrue(right); // FALSE != right -> right must have been true
             } else if (getExpressionId(right) == ExpressionId.Const && getConstValueI32(right) == 0) {
-              this.inheritNonnullIf(left); // left != FALSE
+              this.inheritNonnullIfTrue(left); // left != FALSE -> left must have been true
             }
             break;
           }
@@ -672,9 +679,9 @@ export class Flow {
             let left = getBinaryLeft(expr);
             let right = getBinaryRight(expr);
             if (getExpressionId(left) == ExpressionId.Const && getConstValueI64Low(left) == 0 && getConstValueI64High(left) == 0) {
-              this.inheritNonnullIf(right); // FALSE != right
+              this.inheritNonnullIfTrue(right); // FALSE != right -> right must have been true
             } else if (getExpressionId(right) == ExpressionId.Const && getConstValueI64Low(right) == 0 && getConstValueI64High(right) == 0) {
-              this.inheritNonnullIf(left); // left != FALSE
+              this.inheritNonnullIfTrue(left); // left != FALSE -> left must have been true
             }
             break;
           }
@@ -684,17 +691,15 @@ export class Flow {
     }
   }
 
-  /** Sets local states where this branch is only taken when `expr` is false-ish. */
-  inheritNonnullIfNot(expr: ExpressionRef): void {
+  /** Updates local states to reflect that this branch is only taken when `expr` is false-ish. */
+  inheritNonnullIfFalse(expr: ExpressionRef): void {
+    // A: `expr` is false-ish -> Q: how did that happen?
     switch (getExpressionId(expr)) {
       case ExpressionId.Unary: {
         switch (getUnaryOp(expr)) {
-          case UnaryOp.EqzI32: {
-            this.inheritNonnullIf(getUnaryValue(expr)); // !expr
-            break;
-          }
+          case UnaryOp.EqzI32:
           case UnaryOp.EqzI64: {
-            this.inheritNonnullIf(getUnaryValue(expr)); // !expr
+            this.inheritNonnullIfTrue(getUnaryValue(expr)); // !value -> value must have been true
             break;
           }
         }
@@ -702,25 +707,32 @@ export class Flow {
       }
       case ExpressionId.If: {
         let ifTrue = getIfTrue(expr);
-        if (getExpressionId(ifTrue) == ExpressionId.Const && getExpressionType(ifTrue) == NativeType.I32 && getConstValueI32(ifTrue) != 0) {
+        if (getExpressionId(ifTrue) == ExpressionId.Const) {
           let ifFalse = getIfFalse(expr);
           if (!ifFalse) break;
           // Logical OR: (if (condition 1 ifFalse))
-          // the only way this can become false is if condition and ifFalse are false
-          this.inheritNonnullIfNot(getIfCondition(expr));
-          this.inheritNonnullIfNot(getIfFalse(expr));
+          // the only way this had become false is if condition and ifFalse are false
+          if (
+            (getExpressionType(ifTrue) == NativeType.I32 && getConstValueI32(ifTrue) != 0) ||
+            (getExpressionType(ifTrue) == NativeType.I64 && (getConstValueI64Low(ifTrue) != 0 || getConstValueI64High(ifTrue) != 0))
+          ) {
+            this.inheritNonnullIfFalse(getIfCondition(expr));
+            this.inheritNonnullIfFalse(getIfFalse(expr));
+          }
+
         }
         break;
       }
       case ExpressionId.Binary: {
         switch (getBinaryOp(expr)) {
+          // remember: we want to know how the _entire_ expression became FALSE (!)
           case BinaryOp.EqI32: {
             let left = getBinaryLeft(expr);
             let right = getBinaryRight(expr);
             if (getExpressionId(left) == ExpressionId.Const && getConstValueI32(left) == 0) {
-              this.inheritNonnullIf(right); // FALSE == right
+              this.inheritNonnullIfTrue(right); // FALSE == right -> right must have been true
             } else if (getExpressionId(right) == ExpressionId.Const && getConstValueI32(right) == 0) {
-              this.inheritNonnullIf(left); // left == FALSE
+              this.inheritNonnullIfTrue(left); // left == FALSE -> left must have been true
             }
             break;
           }
@@ -728,9 +740,9 @@ export class Flow {
             let left = getBinaryLeft(expr);
             let right = getBinaryRight(expr);
             if (getExpressionId(left) == ExpressionId.Const && getConstValueI64Low(left) == 0 && getConstValueI64High(left) == 0) {
-              this.inheritNonnullIf(right); // FALSE == right
+              this.inheritNonnullIfTrue(right); // FALSE == right -> right must have been true
             } else if (getExpressionId(right) == ExpressionId.Const && getConstValueI64Low(right) == 0 && getConstValueI64High(right) == 0) {
-              this.inheritNonnullIf(left); // left == FALSE
+              this.inheritNonnullIfTrue(left); // left == FALSE -> left must have been true
             }
             break;
           }
@@ -738,9 +750,9 @@ export class Flow {
             let left = getBinaryLeft(expr);
             let right = getBinaryRight(expr);
             if (getExpressionId(left) == ExpressionId.Const && getConstValueI32(left) != 0) {
-              this.inheritNonnullIf(right); // TRUE != right
+              this.inheritNonnullIfTrue(right); // TRUE != right -> right must have been true
             } else if (getExpressionId(right) == ExpressionId.Const && getConstValueI32(right) != 0) {
-              this.inheritNonnullIf(left); // left != TRUE
+              this.inheritNonnullIfTrue(left); // left != TRUE -> left must have been true
             }
             break;
           }
@@ -748,9 +760,9 @@ export class Flow {
             let left = getBinaryLeft(expr);
             let right = getBinaryRight(expr);
             if (getExpressionId(left) == ExpressionId.Const && (getConstValueI64Low(left) != 0 || getConstValueI64High(left) != 0)) {
-              this.inheritNonnullIf(right); // TRUE != right
+              this.inheritNonnullIfTrue(right); // TRUE != right -> right must have been true for this to become false
             } else if (getExpressionId(right) == ExpressionId.Const && (getConstValueI64Low(right) != 0 || getConstValueI64High(right) != 0)) {
-              this.inheritNonnullIf(left); // left != TRUE
+              this.inheritNonnullIfTrue(left); // left != TRUE -> left must have been true for this to become false
             }
             break;
           }
diff --git a/tests/compiler/possibly-null.optimized.wat b/tests/compiler/possibly-null.optimized.wat
index c83e8f1a..fe8403d3 100644
--- a/tests/compiler/possibly-null.optimized.wat
+++ b/tests/compiler/possibly-null.optimized.wat
@@ -20,6 +20,7 @@
  (export "testLogicalOr" (func $possibly-null/testTrue))
  (export "testLogicalAndMulti" (func $possibly-null/testLogicalAndMulti))
  (export "testLogicalOrMulti" (func $possibly-null/testLogicalAndMulti))
+ (export "testAssign" (func $possibly-null/testLogicalAndMulti))
  (func $possibly-null/testTrue (; 0 ;) (type $FUNCSIG$vi) (param $0 i32)
   nop
  )
diff --git a/tests/compiler/possibly-null.ts b/tests/compiler/possibly-null.ts
index 01af0a4a..101c103f 100644
--- a/tests/compiler/possibly-null.ts
+++ b/tests/compiler/possibly-null.ts
@@ -99,14 +99,23 @@ export function testLogicalAndMulti(a: Ref | null, b: Ref | null): void {
   if (a && b) {
     if (isNullable(a)) ERROR("should be non-nullable");
     if (isNullable(b)) ERROR("should be non-nullable");
+  } else {
+    if (!isNullable(a)) ERROR("should be nullable");
+    if (!isNullable(b)) ERROR("should be nullable");
   }
 }
 
 export function testLogicalOrMulti(a: Ref | null, b: Ref | null): void {
   if (!a || !b) {
-    // something
+    if (!isNullable(a)) ERROR("should be nullable");
+    if (!isNullable(b)) ERROR("should be nullable");
   } else {
     if (isNullable(a)) ERROR("should be non-nullable");
     if (isNullable(b)) ERROR("should be non-nullable");
   }
 }
+
+export function testAssign(a: Ref | null, b: Ref): void {
+  a = b;
+  if (isNullable(a)) ERROR("should be non-nullable");
+}
diff --git a/tests/compiler/possibly-null.untouched.wat b/tests/compiler/possibly-null.untouched.wat
index 5ad30504..a4be7dfb 100644
--- a/tests/compiler/possibly-null.untouched.wat
+++ b/tests/compiler/possibly-null.untouched.wat
@@ -23,6 +23,7 @@
  (export "testLogicalOr" (func $possibly-null/testLogicalOr))
  (export "testLogicalAndMulti" (func $possibly-null/testLogicalAndMulti))
  (export "testLogicalOrMulti" (func $possibly-null/testLogicalOrMulti))
+ (export "testAssign" (func $possibly-null/testAssign))
  (func $possibly-null/testTrue (; 0 ;) (type $FUNCSIG$vi) (param $0 i32)
   local.get $0
   if
@@ -168,6 +169,8 @@
   end
   if
    nop
+  else   
+   nop
   end
  )
  (func $possibly-null/testLogicalOrMulti (; 16 ;) (type $FUNCSIG$vii) (param $0 i32) (param $1 i32)
@@ -185,6 +188,10 @@
    nop
   end
  )
- (func $null (; 17 ;) (type $FUNCSIG$v)
+ (func $possibly-null/testAssign (; 17 ;) (type $FUNCSIG$vii) (param $0 i32) (param $1 i32)
+  local.get $1
+  local.set $0
+ )
+ (func $null (; 18 ;) (type $FUNCSIG$v)
  )
 )