Error: Unable to set the XValues property of the Series class

I am trying to debug code written a couple years ago by someone other than me. The code is below. I am trying to loop through several worksheets and update a chart on each sheet. I get the error, "Unable to set the XValues property of the Series class".

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Here is the line that is the culprit for the error:

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Any ideas of why I might get this error and how I might fix it? Thanks!