diff --git a/js/src/jit-test/tests/gc/bug-1371908.js b/js/src/jit-test/tests/gc/bug-1371908.js
deleted file mode 100644
index da4540459c..0000000000
--- a/js/src/jit-test/tests/gc/bug-1371908.js
+++ /dev/null
@@ -1,6 +0,0 @@
-if (helperThreadCount() === 0)
-    quit();
-offThreadCompileScript("");
-startgc(0);
-runOffThreadScript();
-
diff --git a/js/src/jit-test/tests/gc/bug1146213.js b/js/src/jit-test/tests/gc/bug1146213.js
deleted file mode 100644
index 14ca088275..0000000000
--- a/js/src/jit-test/tests/gc/bug1146213.js
+++ /dev/null
@@ -1,21 +0,0 @@
-if (!("setGCCallback" in this &&
-      "schedulegc" in this &&
-      "gcslice" in this &&
-      "newGlobal" in this &&
-      "helperThreadCount" in this))
-{
-    quit();
-}
-
-if (helperThreadCount() == 0)
-    quit();
-
-setGCCallback({
-  action: "majorGC",
-});
-schedulegc(this)
-gcslice(3)
-var lfGlobal = newGlobal();
-lfGlobal.offThreadCompileScript("");
-lfGlobal.runOffThreadScript();
-
diff --git a/js/src/jsgc.cpp b/js/src/jsgc.cpp
index 73a2169a5f..cf3612d41f 100644
--- a/js/src/jsgc.cpp
+++ b/js/src/jsgc.cpp
@@ -6845,11 +6845,7 @@ gc::MergeCompartments(JSCompartment* source, JSCompartment* target)
 
     JSRuntime* rt = source->runtimeFromMainThread();
 
-    MOZ_ASSERT(!source->zone()->wasGCStarted());
-    MOZ_ASSERT(!target->zone()->wasGCStarted());
-    JS::AutoAssertOnGC nogc(rt);
-
-    AutoTraceSession session(rt);
+    AutoPrepareForTracing prepare(rt, SkipAtoms);
 
     // Cleanup tables and other state in the source compartment that will be
     // meaningless after merging into the target compartment.
diff --git a/js/src/vm/HelperThreads.cpp b/js/src/vm/HelperThreads.cpp
index 564f90b8d1..fda93abfbe 100644
--- a/js/src/vm/HelperThreads.cpp
+++ b/js/src/vm/HelperThreads.cpp
@@ -1103,13 +1103,12 @@ GlobalHelperThreadState::mergeParseTaskCompartment(JSRuntime* rt, ParseTask* par
                                                    Handle<GlobalObject*> global,
                                                    JSCompartment* dest)
 {
-    // Finish any ongoing incremental GC that may affect the destination zone.
-    if (JS::IsIncrementalGCInProgress(rt) && dest->zone()->wasGCStarted())
-        JS::FinishIncrementalGC(rt, JS::gcreason::API);
-
     // After we call LeaveParseTaskZone() it's not safe to GC until we have
     // finished merging the contents of the parse task's compartment into the
-    // destination compartment.
+    // destination compartment.  Finish any ongoing incremental GC first and
+    // assert that no allocation can occur. (Bug 1371908 changes this, but
+    // generates assertion failures.)
+    gc::AutoFinishGC finishGC(rt);
     JS::AutoAssertNoAlloc noAlloc(rt);
 
     LeaveParseTaskZone(rt, parseTask);