diff --git a/src/hotspot/share/classfile/systemDictionary.cpp b/src/hotspot/share/classfile/systemDictionary.cpp
index 9b49035..fbc2587 100644
--- a/src/hotspot/share/classfile/systemDictionary.cpp
+++ b/src/hotspot/share/classfile/systemDictionary.cpp
@@ -1859,6 +1859,7 @@ bool SystemDictionary::do_unloading(GCTimer* gc_timer,
   }
 
   if (unloading_occurred) {
+    MutexLockerEx ml(UseZGC ? SystemDictionary_lock : NULL);
     GCTraceTime(Debug, gc, phases) t("Dictionary", gc_timer);
     constraints()->purge_loader_constraints();
     resolution_errors()->purge_resolution_errors();