diff --git a/src/vs/editor/common/model/textModel.ts b/src/vs/editor/common/model/textModel.ts
index aac822b74af24fcee92d55a48b4739bb723a4d75..f59d9c0d7bf97bc00006147d02bf2771afdf38db 100644
--- a/src/vs/editor/common/model/textModel.ts
+++ b/src/vs/editor/common/model/textModel.ts
@@ -2757,7 +2757,7 @@ class DecorationsTrees {
 }
 
 function cleanClassName(className: string): string {
-	return className.replace(/[^a-z0-9\-]/gi, ' ');
+	return className.replace(/[^a-z0-9\-_]/gi, ' ');
 }
 
 export class ModelDecorationOverviewRulerOptions implements model.IModelDecorationOverviewRulerOptions {