"Invalid Call or Argument" in While Loop using MyFile = Dir

Hi guys,
When running the following code, once all the files have been looped through, I occasionally get an "Invalid call or argument" error at the bottom "myFile = Dir" line. It happens only sometimes, and other times the code executes fine.
What could I be missing? I know without much more information its a bit vague, but where would you suggest I investigate as to the cause? I'm just about to stick On Error Resume Next above it, since it only ever occurs after all the workbooks have been looped through and the loop is completed anyway - But thats lazy... any idea?

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The only times I use "Dir" are already shown in the original post. But below is the full code inside the loop....

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