--- a/xpcom/base/nsCycleCollector.cpp
+++ b/xpcom/base/nsCycleCollector.cpp
@@ -3278,16 +3278,17 @@ nsCycleCollector::CollectWhite()
   nsCycleCollectionParticipant* zoneParticipant =
     hasJSRuntime ? mJSRuntime->ZoneParticipant() : nullptr;
 
   NodePool::Enumerator etor(mGraph.mNodes);
   while (!etor.IsDone()) {
     PtrInfo* pinfo = etor.GetNext();
     if (pinfo->mColor == white && pinfo->mParticipant) {
       if (pinfo->IsGrayJS()) {
+        MOZ_ASSERT(mJSRuntime);
         ++numWhiteGCed;
         JS::Zone* zone;
         if (MOZ_UNLIKELY(pinfo->mParticipant == zoneParticipant)) {
           ++numWhiteJSZones;
           zone = static_cast<JS::Zone*>(pinfo->mPointer);
         } else {
           JS::GCCellPtr ptr(pinfo->mPointer, js::GCThingTraceKind(pinfo->mPointer));
           zone = JS::GetTenuredGCThingZone(ptr);