diff --git a/lib/codemirror/cm.defs b/lib/codemirror/cm.defs
index a9db084bb5..edd8dd6c29 100644
--- a/lib/codemirror/cm.defs
+++ b/lib/codemirror/cm.defs
@@ -23,6 +23,7 @@ CM_ADDONS = [
   'search/searchcursor.js',
   'search/search.js',
   'selection/mark-selection.js',
+  'mode/multiplex.js',
   'mode/overlay.js',
   'mode/simple.js',
 ]