diff --git a/lib/src/source.rs b/lib/src/source.rs
index e7afcc0..61f0209 100644
--- a/lib/src/source.rs
+++ b/lib/src/source.rs
@@ -261,9 +261,9 @@ pub mod reader_sync {
             zip_file
                 .start_file("test.json", options)
                 .expect("zip start file");
-            zip_file
-                .write(serde_json::to_vec(&batch).unwrap().as_slice())
-                .expect("write entry");
+            let data = serde_json::to_vec(&batch).unwrap();
+            let amount = zip_file.write(data.as_slice()).expect("write entry");
+            assert_eq!(amount, data.len());
             zip_file.finish().expect("finish zip file");
         }