diff --git a/xpcom/glue/nsProxyRelease.h b/xpcom/glue/nsProxyRelease.h
index 51a330c04e8f..1c53aa75f441 100644
--- a/xpcom/glue/nsProxyRelease.h
+++ b/xpcom/glue/nsProxyRelease.h
@@ -165,6 +165,7 @@ class nsMainThreadPtrHandle
   nsMainThreadPtrHandle(const nsMainThreadPtrHandle& aOther) : mPtr(aOther.mPtr) {}
   nsMainThreadPtrHandle& operator=(const nsMainThreadPtrHandle& aOther) {
     mPtr = aOther.mPtr;
+    return *this;
   }
 
   operator nsMainThreadPtrHolder<T>*() { return mPtr.get(); }