DAX issue with Date data type

I am having an issue today with a DAX formula.

The scenario is as follows. I have 2 Power BI desktop files which are formatted identically. I have them linked to separate Excel workbooks which are formatted identically. I am inserting a calculated column into both datasets under "Modeling" in Power BI with the following formula:

Week_Crctd = if(weeknum('Line Calls'[Date])<=52,(weeknum('Line Calls'[Date])),1)

In one PBI desktop file, I receive the expected result; a whole number between 1 and 52 which varies correctly according to the [Date] value. In the other file, I receive the following error:

As I mentioned before, both Excel files are formatted with the exact same number formatting in these columns and both PBI desktop files are formatted with the same Data Type, Format, Data Category, and Default Summarization. What gives?

Thanks guys and gals