diff --git a/libraries/script-engine/src/ScriptEngines.cpp b/libraries/script-engine/src/ScriptEngines.cpp
index d78fd9fe0c..61220bc00b 100644
--- a/libraries/script-engine/src/ScriptEngines.cpp
+++ b/libraries/script-engine/src/ScriptEngines.cpp
@@ -119,6 +119,7 @@ void ScriptEngines::registerScriptInitializer(ScriptInitializer initializer) {
 
 void ScriptEngines::addScriptEngine(ScriptEngine* engine) {
     if (!_stoppingAllScripts) {
+        QMutexLocker locker(&_allScriptsMutex);
         _allKnownScriptEngines.insert(engine);
     }
 }