diff --git a/tools/releasetools/sparse_img.py b/tools/releasetools/sparse_img.py
index 51a1643..49b3a67 100644
--- a/tools/releasetools/sparse_img.py
+++ b/tools/releasetools/sparse_img.py
@@ -125,7 +125,7 @@
     clobbered_blocks."""
     ranges = self.care_map
     if not include_clobbered_blocks:
-      ranges.subtract(self.clobbered_blocks)
+      ranges = ranges.subtract(self.clobbered_blocks)
     h = sha1()
     for d in self._GetRangeData(ranges):
       h.update(d)