Reading a folder of XLS files with different column numbers

I'm trying to concatenate many .xls files in a folder into one table.

I've tried using List Files -> Table Row to Variable Loop Start -> XLS Reader -> Loop End (and assigning XLS_LOCATION in the 'Flow Variables' tab of the XLS Reader to 'Location') but I get an error saying:

ERROR Loop End: Execute failed: Input table's structure differs from reference (first iteration) table: different column counts 28 vs 32.

How could I solve this problem?