diff --git a/js/src/jscompartment.cpp b/js/src/jscompartment.cpp
index 680dd096ef0..a3c349c4d52 100644
--- a/js/src/jscompartment.cpp
+++ b/js/src/jscompartment.cpp
@@ -432,6 +432,7 @@ JSCompartment::wrap(JSContext *cx, JSObject **objp, JSObject *existingArg)
 bool
 JSCompartment::wrapId(JSContext *cx, jsid *idp)
 {
+    MOZ_ASSERT(*idp != JSID_VOID, "JSID_VOID is an out-of-band sentinel value");
     if (JSID_IS_INT(*idp))
         return true;
     RootedValue value(cx, IdToValue(*idp));