Add blank payment type

Under Payment info, I usually select charge to bill out separate departments at our organization. For our own department we leave this field blank and everything is posted as an internal cost. Once in awhile out of habit I'll select a payment type and then catch my error but am unable to clear this field once a type has been selected. It would be of great help to be able to blank/clear the "payment type" field without having to void the work order and build it again from scratch.