--- a/js/src/gc/Cell.h
+++ b/js/src/gc/Cell.h
@@ -388,17 +388,17 @@ TenuredCell::readBarrier(TenuredCell* th
         Cell* tmp = thing;
         TraceManuallyBarrieredGenericPointerEdge(shadowZone->barrierTracer(), &tmp, "read barrier");
         MOZ_ASSERT(tmp == thing);
     }
 
     if (thing->isMarkedGray()) {
         // There shouldn't be anything marked grey unless we're on the main thread.
         MOZ_ASSERT(CurrentThreadCanAccessRuntime(thing->runtimeFromAnyThread()));
-        if (!RuntimeFromMainThreadIsHeapMajorCollecting(shadowZone))
+        if (!JS::CurrentThreadIsHeapCollecting())
             JS::UnmarkGrayGCThingRecursively(JS::GCCellPtr(thing, thing->getTraceKind()));
     }
 }
 
 void
 AssertSafeToSkipBarrier(TenuredCell* thing);
 
 /* static */ MOZ_ALWAYS_INLINE void

new file mode 100644
--- /dev/null
+++ b/js/src/tests/non262/regress/regress-1456512-greyreadbarrier.js
@@ -0,0 +1,7 @@
+var wm = new WeakMap();
+grayRoot().map = wm;
+wm = null;
+gczeal(13, 7);
+var lfOffThreadGlobal = newGlobal();
+
+reportCompare('do not crash', 'do not crash', 'did not crash!');