diff --git a/build/src/main.rs b/build/src/main.rs
index 437f795..4845009 100644
--- a/build/src/main.rs
+++ b/build/src/main.rs
@@ -14,7 +14,7 @@ use std::path::PathBuf;
 use clap::{App, Arg};
 use parity_wasm::elements;
 
-use wasm_utils::{CREATE_SYMBOL, CALL_SYMBOL, MEMORY_SYMBOL, ununderscore_funcs};
+use wasm_utils::{CREATE_SYMBOL, CALL_SYMBOL, MEMORY_SYMBOL, ununderscore_funcs, externalize_mem};
 
 #[derive(Debug)]
 pub enum Error {
@@ -128,6 +128,10 @@ fn main() {
 		module = ununderscore_funcs(module);
 	}
 
+	if let source::SourceTarget::Unknown = source_input.target() {
+		module = externalize_mem(module);
+	}
+
 	if let Some(runtime_type) = matches.value_of("runtime_type") {
 		let runtime_type: &[u8] = runtime_type.as_bytes();
 		if runtime_type.len() != 4 {