In case anyone comes across this in the future, here is the solution:
I saw in the data output file that PsychoPy was trying to start an 11th trial, even though I had only put 10 trials into the Excel sheet that was being read in. I came across this discussion, so I exported my Excel file to a .csv and opened it in TextEdit. There were about a dozen empty rows, denoted by a lot of commas. I deleted every “empty” row and column in my spreadsheet (control+shift+down arrow), resaved it as an Excel file, and that seems to have fixed the problem.