diff --git a/.gitattributes b/.gitattributes
index a17611cd..4583b202 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -1,3 +1,4 @@
 bin/asc text eol=lf
 dist/asc.js -diff
 dist/assemblyscript.js -diff
+scripts/check-pr.sh eol=lf
diff --git a/.travis.yml b/.travis.yml
index d3cf2688..6b5f9f69 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,17 @@
 language: node_js
+notifications:
+  email: false
+stages:
+  - name: check-pr
+    if: type = pull_request
 jobs:
   include:
 
+  - stage: check-pr
+    node_js: lts/*
+    script: ./scripts/check-pr.sh
+    env: Checks contributing guidelines before testing pull requests
+
   - stage: lint
     node_js: node
     script: npm run lint
diff --git a/scripts/check-pr.sh b/scripts/check-pr.sh
new file mode 100755
index 00000000..1657439c
--- /dev/null
+++ b/scripts/check-pr.sh
@@ -0,0 +1,29 @@
+# Distribution files should not be modified
+STATUS=0
+if git --no-pager diff --name-only FETCH_HEAD $(git merge-base FETCH_HEAD $TRAVIS_BRANCH) | grep -q "^dist/"; then
+  STATUS=1 &&
+  printf "\n" &&
+  printf "The pull request includes changes to distribution files, but it shouldn't.\n" &&
+  printf "Please see https://github.com/AssemblyScript/assemblyscript/blob/master/CONTRIBUTING.md\n";
+else
+  printf "\n" &&
+  printf "GOOD: The pull request does not include changes to distribution files.\n";
+fi
+
+# Authors should have added themself to the NOTICE file
+AUTHOR=$(git log -1 --format="%aE")
+if [ -z "$AUTHOR" ]; then
+  printf "\n" &&
+  printf "Skipping NOTICE check: Commit does not include an email address.\n";
+else
+  if grep -q "$AUTHOR" NOTICE; then
+    printf "\n" &&
+    printf "GOOD: Author is present in the NOTICE file.\n";
+  else
+    printf "\n" &&
+    printf "Author does not appear to be listed in the NOTICE file, yet.\n" &&
+    printf "Please see https://github.com/AssemblyScript/assemblyscript/blob/master/CONTRIBUTING.md\n";
+  fi
+fi
+
+exit $STATUS
diff --git a/src/types.ts b/src/types.ts
index 94b93b4d..c410e126 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -541,7 +541,7 @@ export class Signature {
     // check return type
     var thisReturnType = this.returnType;
     var targetReturnType = target.returnType;
-    return thisReturnType == targetReturnType || this.returnType.isAssignableTo(target.returnType);
+    return thisReturnType == targetReturnType || thisReturnType.isAssignableTo(targetReturnType);
   }
 
   /** Converts this signature to a function type string. */