diff --git a/server/core/Reader/BookConverter/ConvertBase.js b/server/core/Reader/BookConverter/ConvertBase.js
index b2a5f712..0c21bd12 100644
--- a/server/core/Reader/BookConverter/ConvertBase.js
+++ b/server/core/Reader/BookConverter/ConvertBase.js
@@ -34,6 +34,8 @@ class ConvertBase {
     }
 
     async execConverter(path, args, onData, abort) {
+        onData = (onData ? onData : () => {});
+        
         let q = null;
         try {
             q = await queue.get(() => {onData();});