Error in sum function when using merged cells

[librarynut]

Help! I recently discovered an error in my shipping worksheet. I sometimes need to merge certain cells when I am combining items from different purchase orders into the same shipment. However, when I totaled the column (SUM) containing the merged cells, it delivers a wrong answer.

Specifically column F and column H that contain the merged cells don't add up correctly. Column F is +1 more than it should be. Column H is +125 more than it should be.

I have been using this feature for about 6 months, and this is the first time it did this. Any ideas why the error?