diff --git a/lld/test/wasm/lto/weak-undefined.ll b/lld/test/wasm/lto/weak-undefined.ll
--- a/lld/test/wasm/lto/weak-undefined.ll
+++ b/lld/test/wasm/lto/weak-undefined.ll
@@ -11,6 +11,17 @@
 
 declare extern_weak i32 @foo()
 
+declare extern_weak i32 @bar()
+
+; The refernce to bar here will exist in the LTO file, but the LTO process will
+; remove it, so it will never be referenced by an normal object file, and
+; therefore never have a signature.
+define void @unused_function() #0 {
+entry:
+    %call2 = call i32 @bar()
+    ret void
+}
+
 define void @_start() #0 {
 entry:
     %call2 = call i32 @foo()
diff --git a/lld/wasm/SymbolTable.cpp b/lld/wasm/SymbolTable.cpp
--- a/lld/wasm/SymbolTable.cpp
+++ b/lld/wasm/SymbolTable.cpp
@@ -842,7 +842,7 @@
 void SymbolTable::replaceWithUndefined(Symbol *sym) {
   // Add a synthetic dummy for weak undefined functions.  These dummies will
   // be GC'd if not used as the target of any "call" instructions.
-  StringRef debugName = saver().save("undefined_weak:" + toString(*sym));
+  StringRef debugName = saver().save("undefined_weak:" + sym->getName());
   replaceWithUnreachable(sym, *sym->getSignature(), debugName);
   // Hide our dummy to prevent export.
   sym->setHidden(true);
@@ -854,7 +854,7 @@
 // the call instruction that passes Wasm validation.
 void SymbolTable::handleWeakUndefines() {
   for (Symbol *sym : getSymbols()) {
-    if (sym->isUndefWeak()) {
+    if (sym->isUndefWeak() && sym->isUsedInRegularObj) {
       if (sym->getSignature()) {
         replaceWithUndefined(sym);
       } else {