#44 unique invoice number on suspend sale

[Anonymous]

when trying to suspend a sale i get the warning "Please enter an unique invoice number" even though the invoice check button is not checked. The default value entered is 0, so if there is already a suspended sale, that one get invoice 0, the next sale to get suspended trys to take a zero invoice as well, so it fails with the error message.
Changing the invoice number to a random number will enable the sale to suspend, but as the invoice number checkbox is not checked this is non obvious