--- a/js/src/gc/Marking.cpp
+++ b/js/src/gc/Marking.cpp
@@ -1014,17 +1014,17 @@ GCMarker::processMarkStackOther(SliceBud
                 JS_ASSERT(thing <= arena->thingsEnd());
                 if (thing == span->first) {
                     if (!span->hasNext())
                         break;
                     thing = span->last;
                     span = span->nextSpan();
                 } else {
                     JSObject *object = reinterpret_cast<JSObject *>(thing);
-                    if (object->hasSingletonType())
+                    if (object->hasSingletonType() && object->markIfUnmarked(getMarkColor()))
                         pushObject(object);
                     budget.step();
                 }
             }
             if (budget.isOverBudget()) {
                 pushArenaList(aheader);
                 return;
             }