Debug Problem, possibly involving sheet names?

Alright, I have a function called ListSheetNames() stored in a sheet named ProjectView (this is the VBA code name, it is titled "Project View" in Excel). I know that all the calls within ListSheetNames() work correctly.

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If I run this function from a button in Excel, assigned to ProjectView.ListSheetNames, then it works properly and runs error-free.

However, if I try to call the function as the workbook opens via:

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I get a run-time error 1004, highlighting "Call ProjectView.ListSheetNames"

Alternatively, if I call the function with the following function on another sheet:

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The call to Tester works properly, the call to ListSheetNames generates the list of sheet names as intended, and it yields a 400 error.

Does anyone have any clue what's going on? My ListSheetNames function works if I run it from a button in Excel, but it gets errors when I call it from other functions. What gives?