diff --git a/src/services/shims.ts b/src/services/shims.ts
index aeb35497876..307062cebd8 100644
--- a/src/services/shims.ts
+++ b/src/services/shims.ts
@@ -1097,4 +1097,4 @@ module TypeScript.Services {
 }
 
 /* @internal */
-let toolsVersion = "1.6";
+const toolsVersion = "1.6";