diff --git a/amitools/fs/block/BootBlock.py b/amitools/fs/block/BootBlock.py
index ad09a957..d1c11638 100644
--- a/amitools/fs/block/BootBlock.py
+++ b/amitools/fs/block/BootBlock.py
@@ -52,9 +52,9 @@ def _calc_chksum(self):
         all_blks = [self] + self.extra_blks
         n = self.blkdev.block_longs
         chksum = 0
-        for blk in all_blks:
+        for b, blk in enumerate(all_blks):
             for i in range(n):
-                if i != 1:  # skip chksum
+                if not (b == 0 and i == 1):  # skip chksum
                     chksum += blk._get_long(i)
                     if chksum > 0xFFFFFFFF:
                         chksum += 1