diff --git a/tools/releasetools/blockimgdiff.py b/tools/releasetools/blockimgdiff.py
index 25d2743..625dca2 100644
--- a/tools/releasetools/blockimgdiff.py
+++ b/tools/releasetools/blockimgdiff.py
@@ -771,7 +771,10 @@
           x = x.subtract(sr)
 
       for s, e in x:
-        for i in range(s, e):
+        # Source image could be larger. Don't check the blocks that are in the
+        # source image only. Since they are not in 'touched', and won't ever
+        # be touched.
+        for i in range(s, min(e, self.tgt.total_blocks)):
           assert touched[i] == 0
 
       # Check that the output blocks for this transfer haven't yet