diff --git a/payload_generator/delta_diff_utils.cc b/payload_generator/delta_diff_utils.cc
index de6a23d..137a77b 100644
--- a/payload_generator/delta_diff_utils.cc
+++ b/payload_generator/delta_diff_utils.cc
@@ -183,8 +183,8 @@
       aops,
       old_part.path,
       new_part.path,
-      old_part.fs_interface ? old_part.fs_interface->GetBlockCount() : 0,
-      new_part.fs_interface->GetBlockCount(),
+      old_part.size / kBlockSize,
+      new_part.size / kBlockSize,
       soft_chunk_blocks,
       version,
       blob_file,