diff --git a/payload_generator/cow_size_estimator.cc b/payload_generator/cow_size_estimator.cc
index fc66e67..01e9965 100644
--- a/payload_generator/cow_size_estimator.cc
+++ b/payload_generator/cow_size_estimator.cc
@@ -116,6 +116,7 @@
   cow_writer.Initialize(android::base::borrowed_fd{-1});
   CHECK(CowDryRun(
       target_fd, operations, merge_operations, block_size, &cow_writer));
+  CHECK(cow_writer.Finalize());
   return cow_writer.GetCowSize();
 }