diff --git a/js/src/gc/Marking.cpp b/js/src/gc/Marking.cpp
index b675cf4ba2cc..8dc3539df4c3 100644
--- a/js/src/gc/Marking.cpp
+++ b/js/src/gc/Marking.cpp
@@ -3504,6 +3504,10 @@ struct AssertNonGrayTracer final : public JS::CallbackTracer {
   void onChild(const JS::GCCellPtr& thing) override {
     MOZ_ASSERT(!thing.asCell()->isMarkedGray());
   }
+  // This is used by the UnmarkGray tracer only, and needs to report itself
+  // as the non-gray tracer to not trigger assertions.  Do not use it in another
+  // context without making this more generic.
+  TracerKind getTracerKind() const override { return TracerKind::UnmarkGray; }
 };
 #endif
 
diff --git a/js/src/jit-test/tests/gc/bug-1557928.js b/js/src/jit-test/tests/gc/bug-1557928.js
new file mode 100644
index 000000000000..560df858e99a
--- /dev/null
+++ b/js/src/jit-test/tests/gc/bug-1557928.js
@@ -0,0 +1,16 @@
+var g = newGlobal({ newCompartment: true });
+var dbg = new Debugger;
+var gw = dbg.addDebuggee(g);
+lfOffThreadGlobal = g;
+lfOffThreadGlobal.offThreadCompileScript(`
+    grayRoot()[0] = "foo";
+  `);
+lfOffThreadGlobal.runOffThreadScript();
+var g = newGlobal({newCompartment: true});
+var gw = dbg.addDebuggee(g);
+lfOffThreadGlobal = null;
+gc();
+schedulegc(this);
+schedulegc('atoms');
+gc('zone');
+