The Excel input file likely contains one or more blank/empty columns directly to the right of the last column that contains data. As a test, delete columns to the right of the last populated column, and save the Excel file. Then, try running your process again to see if it works. If so, you've found the root cause of the issue, and you must identify and resolve why it is happening. Is the input Excel file generated by some upstream manual or automated process that can be the cause of the empty columns? If you cannot find the cause and must continue to use the input file as is, you must predefine the column names in the collection in order for it to work.