diff --git a/source-map/source-map.d.ts b/source-map/source-map.d.ts
index 3ddd49b537..016798d913 100644
--- a/source-map/source-map.d.ts
+++ b/source-map/source-map.d.ts
@@ -73,7 +73,7 @@ declare module SourceMap {
         constructor(line: number, column: number, source: string);
         constructor(line: number, column: number, source: string, chunk?: string, name?: string);
         public static fromStringWithSourceMap(code: string, sourceMapConsumer: SourceMapConsumer, relativePath?: string): SourceNode;
-        public add(chunk: string): void;
+        public add(chunk: any): SourceNode;
         public prepend(chunk: string): void;
         public setSourceContent(sourceFile: string, sourceContent: string): void;
         public walk(fn: (chunk: string, mapping: MappedPosition) => void): void;