diff --git a/packages/compiler-cli/src/perform_compile.ts b/packages/compiler-cli/src/perform_compile.ts
index 2f53d37949..18fc69b9eb 100644
--- a/packages/compiler-cli/src/perform_compile.ts
+++ b/packages/compiler-cli/src/perform_compile.ts
@@ -43,7 +43,7 @@ export function formatDiagnostics(options: api.CompilerOptions, diags: Diagnosti
             return res;
           }
         })
-        .join();
+        .join('');
   } else
     return '';
 }