--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -3571,16 +3571,17 @@ WorkerMain(void* arg)
         return;
 
     ShellContext* sc = js_new<ShellContext>(cx);
     if (!sc)
         return;
 
     auto guard = mozilla::MakeScopeExit([&] {
         CancelOffThreadJobsForContext(cx);
+        sc->markObservers.reset();
         JS_DestroyContext(cx);
         js_delete(sc);
         js_delete(input);
     });
 
     sc->isWorker = true;
     JS_SetContextPrivate(cx, sc);
     SetWorkerContextOptions(cx);

new file mode 100644
--- /dev/null
+++ b/js/src/tests/non262/regress/regress-1456518-workergray.js
@@ -0,0 +1,7 @@
+if (typeof 'evalInWorder' == 'function') {
+    evalInWorker(`
+  addMarkObservers([grayRoot(), grayRoot().x, this, Object.create(null)]);
+`);
+}
+
+reportCompare('do not crash', 'do not crash', 'did not crash!');