Code "debug" error msg

The two code extracts below are associated with two separate worksheets in the same workbook. They work fine. They simply are used to assign either a PO# or a Temp Unit #, located in a third sheet, in their respective sheets.

The problem is that when I'm in either the Master or the Work Orders sheets where these two codes are used, and I do something simple like drag down some values or paste something in a cell, I get the "Run time error "13" type mismatch error" in the popup debug window for my VBA code. And in each case, it highlights the codeline I've color coded below:

Work Orders Code:

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Master Code

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My problem is I'm so new to this type of coding that I'm totally lost as to what it is really telling me or what to do. It doesn't keep me from doing what I need to do nor does it interfere with what the code does..I just don't know how to fix it.

I've attached the sample spreadsheet.

If you uncomment out those first lines, it will probably stop doing that. By having the the apostrophe it turns the first line of code OFF (you see it as green text in the editor).

Take out the apostrophes. See if it behaves better.

You should also avoid copy or dragging across those columns that have the macros watching them anyway.