diff --git a/tools/releasetools/blockimgdiff.py b/tools/releasetools/blockimgdiff.py
index 87cc72b..ded34b9 100644
--- a/tools/releasetools/blockimgdiff.py
+++ b/tools/releasetools/blockimgdiff.py
@@ -723,6 +723,7 @@
     num_of_bytes = new_blocks * self.tgt.blocksize
     print("  Total %d blocks (%d bytes) are packed as new blocks due to "
           "insufficient cache size." % (new_blocks, num_of_bytes))
+    return new_blocks
 
   def ComputePatches(self, prefix):
     print("Reticulating splines...")