diff --git a/tools/releasetools/blockimgdiff.py b/tools/releasetools/blockimgdiff.py
index 72f065d..8b6a690 100644
--- a/tools/releasetools/blockimgdiff.py
+++ b/tools/releasetools/blockimgdiff.py
@@ -1558,6 +1558,7 @@
     split_large_apks = []
     cache_size = common.OPTIONS.cache_size
     split_threshold = 0.125
+    assert cache_size is not None
     max_blocks_per_transfer = int(cache_size * split_threshold /
                                   self.tgt.blocksize)
     empty = RangeSet()