diff --git a/tools/releasetools/blockimgdiff.py b/tools/releasetools/blockimgdiff.py
index 456f791..e5a9050 100644
--- a/tools/releasetools/blockimgdiff.py
+++ b/tools/releasetools/blockimgdiff.py
@@ -84,6 +84,7 @@
     self.extended = RangeSet()
     self.total_blocks = 0
     self.file_map = {}
+    self.hashtree_info = None
 
   def RangeSha1(self, ranges):
     return sha1().hexdigest()