--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -12341,20 +12341,20 @@ class CGDictionary(CGThing):
 
                 """,
                 dictName=self.makeClassName(self.dictionary.parent))
         else:
             body += dedent(
                 """
                 { // scope for isConvertible
                   bool isConvertible;
-                  if (!IsConvertibleToDictionary(cx, val, &isConvertible)) {
+                  if (cx && !IsConvertibleToDictionary(cx, val, &isConvertible)) {
                     return false;
                   }
-                  if (!isConvertible) {
+                  if (cx && !isConvertible) {
                     return ThrowErrorMessage(cx, MSG_NOT_DICTIONARY, sourceDescription);
                   }
                 }
 
                 """)
 
         memberInits = [self.getMemberConversion(m).define()
                        for m in self.memberInfo]