Missing field in Report View

I’ve got an Int field that shows jsut fine in List View. When I switch over to Report View, however, it’s not there. In … Pick Columns, the column doesn’t appear as an option. ‘Report Hide’ is not checked for the field. System Manager gets the same result so I don’t think it’s any kind of permission issue.

Why might a field not show as an option in Report View → Pick Columns?