diff --git a/js/src/vm/HelperThreads.cpp b/js/src/vm/HelperThreads.cpp
index 17768a5bc8f5..4fc560fd8488 100644
--- a/js/src/vm/HelperThreads.cpp
+++ b/js/src/vm/HelperThreads.cpp
@@ -1497,7 +1497,7 @@ LeaveParseTaskZone(JSRuntime* rt, ParseTask* task)
 {
     // Mark the zone as no longer in use by a helper thread, and available
     // to be collected by the GC.
-    rt->clearUsedByHelperThread(task->parseGlobal->zone());
+    rt->clearUsedByHelperThread(task->parseGlobal->zoneFromAnyThread());
 }
 
 ParseTask*
diff --git a/js/src/vm/Runtime.cpp b/js/src/vm/Runtime.cpp
index 4047e677722e..f883463e410d 100644
--- a/js/src/vm/Runtime.cpp
+++ b/js/src/vm/Runtime.cpp
@@ -914,11 +914,12 @@ js::CurrentThreadCanAccessRuntime(const JSRuntime* rt)
 bool
 js::CurrentThreadCanAccessZone(Zone* zone)
 {
-    if (CurrentThreadCanAccessRuntime(zone->runtime_))
-        return true;
+    // Helper thread zones can only be used by their owning thread.
+    if (zone->usedByHelperThread())
+        return zone->group()->ownedByCurrentThread();
 
-    // Only zones marked for use by a helper thread can be used off thread.
-    return zone->usedByHelperThread() && zone->group()->ownedByCurrentThread();
+    // Other zones can only be accessed by the runtime's active context.
+    return CurrentThreadCanAccessRuntime(zone->runtime_);
 }
 
 #ifdef DEBUG