Unable to change to No default column for sorting on existing table.

Whenever I try and change an existing table that currently has a default sorting column to No default column for sorting I get the following error:

Array ( [name] => Electronics [desc] => [order] => desc [default] => NULL [column] => Array ( [0] => Array ( [data] => Offer End Date [required] => 1 [field] => 1 [weight] => 0 ) [1] => Array ( [data] => Product [required] => 1 [weight] => 1 ) [2] => Array ( [data] => Shop [required] => 1 [weight] => 2 ) [3] => Array ( [data] => URL [required] => 1 [weight] => 3 ) [4] => Array ( [data] => Other Details [weight] => 4 ) ) [tid] => 2 [op] => Save [form_build_id] => form-f44257fa031d55b0e2c528a442cc265c [form_token] => 65710c1e0bff0b81cf4f3fa5dc014b04 [form_id] => tablemanager_table_edit )

The same happens if I try and change a required field to not required. Any ideas?

Regards