diff --git a/src/plugins/zip.ts b/src/plugins/zip.ts
index a8b6643ad..8dabc12ba 100644
--- a/src/plugins/zip.ts
+++ b/src/plugins/zip.ts
@@ -35,6 +35,6 @@ export class Zip {
     successIndex: 2,
     errorIndex: 4
   })
-  static unzip(sourceZip: string, destUrl: string, onProgress: Function): Promise<number> {return; }
+  static unzip(sourceZip: string, destUrl: string, onProgress?: Function): Promise<number> {return; }
 
 }