diff --git a/js/src/gc/Marking.cpp b/js/src/gc/Marking.cpp
index 4487594f97a8..4a0c68f4fd91 100644
--- a/js/src/gc/Marking.cpp
+++ b/js/src/gc/Marking.cpp
@@ -1662,6 +1662,18 @@ ObjectDenseElementsMayBeMarkable(NativeObject* nobj)
     return mayBeMarkable;
 }
 
+static inline void
+CheckForCompartmentMismatch(JSObject* obj, JSObject* obj2)
+{
+#ifdef DEBUG
+    if (MOZ_UNLIKELY(obj->compartment() != obj2->compartment())) {
+        fprintf(stderr, "Compartment mismatch in pointer from %s object slot to %s object\n",
+                obj->getClass()->name, obj2->getClass()->name);
+        MOZ_CRASH("Compartment mismatch");
+    }
+#endif
+}
+
 inline void
 GCMarker::processMarkStackTop(SliceBudget& budget)
 {
@@ -1732,7 +1744,7 @@ GCMarker::processMarkStackTop(SliceBudget& budget)
             traverseEdge(obj, v.toString());
         } else if (v.isObject()) {
             JSObject* obj2 = &v.toObject();
-            MOZ_ASSERT(obj->compartment() == obj2->compartment());
+            CheckForCompartmentMismatch(obj, obj2);
             if (mark(obj2)) {
                 // Save the rest of this value array for later and start scanning obj2's children.
                 pushValueArray(obj, vp, end);