diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 4e5ca9f07e77f..7a120064a64cb 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2442,7 +2442,7 @@ namespace ts {
     }
 
     export interface WriteFileCallback {
-        (fileName: string, data: string, writeByteOrderMark: boolean, onError?: (message: string) => void, sourceFiles?: ReadonlyArray<SourceFile>): void;
+        (fileName: string, data: string, writeByteOrderMark: boolean, onError: ((message: string) => void) | undefined, sourceFiles: ReadonlyArray<SourceFile>): void;
     }
 
     export class OperationCanceledException { }