--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -12316,16 +12316,17 @@ class CGDictionary(CGThing):
         if memberInits:
             body += fill(
                 """
                 bool isNull = val.isNullOrUndefined();
                 // We only need these if !isNull, in which case we have |cx|.
                 Maybe<JS::Rooted<JSObject *> > object;
                 Maybe<JS::Rooted<JS::Value> > temp;
                 if (!isNull) {
+                  MOZ_ASSERT(cx);
                   object.emplace(cx, &val.toObject());
                   temp.emplace(cx);
                 }
                 $*{memberInits}
                 """,
                 memberInits="\n".join(memberInits))
 
         body += "return true;\n"