Error submitting sales invoice

When I try to submit a sales invoice I am getting an error to say that a linked timesheet has already been completed. If I try to cancel the timesheet to allow me to submit the sales inovice I get an error because the timesheet is linked to other documents. What is the best solution here and why does this happen? I believe sales invoices should be able to be submitted after a timesheet has been completed as the timesheet would more than not be done first anyway.