diff --git a/src/compiler/sourcemap.ts b/src/compiler/sourcemap.ts
index 3e8e9a1f46c..f77d7c1a2ed 100644
--- a/src/compiler/sourcemap.ts
+++ b/src/compiler/sourcemap.ts
@@ -271,7 +271,7 @@ namespace ts {
                 sourceRoot,
                 sources,
                 names,
-                mappings: mappings.join(),
+                mappings: mappings.join(""),
                 sourcesContent,
             };
         }