--- a/xpcom/io/nsEscape.cpp
+++ b/xpcom/io/nsEscape.cpp
@@ -441,17 +441,19 @@ NS_EscapeAndFilterURL(const nsACString& 
   bool appended = false;
   nsresult rv = T_EscapeURL(aStr.Data(), aStr.Length(), aFlags, aFilterMask, aResult, appended);
   if (NS_FAILED(rv)) {
     aResult.Truncate();
     return rv;
   }
 
   if (!appended) {
-    aResult = aStr;
+    if (!aResult.Assign(aStr, fallible)) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
   }
 
   return rv;
 }
 
 const nsAString&
 NS_EscapeURL(const nsAString& aStr, uint32_t aFlags, nsAString& aResult)
 {