[SOLVED] SHA1 checksum error 2

I got this in my "WinReducerEX81.log".

***** 4/2/2016 3:25:10 PM - SESSION OPENED *****
ERROR : 4/2/2016 3:29:42 PM ==> Optional - Windows8.1-KB3121261-v2-x64.msu = SHA1 value error detected ... This update has been deleted to avoid integration troubles
***** 4/2/2016 3:45:55 PM - SESSION CLOSED *****

I dont understand though.  How is the checksum calculated?
Is the checksum stored in the updates catalog, and then WX81 checks the recently downloaded file?
If that is the case, then I shouldnt trust the file.

Hello,

In fact, I made a little mistake for this file ... so I've already uploaded a fix for the catalog updates file.

Please let me know if that solved the trouble.

It is good now.  Thanks winterstorm.