Unable to create a negative invoice

Hi,

I currently need to credit a customer some money from an invoice but when I go to do this, if the invoice total is more than -$100 it won't allow me to record it and I get the attached error.

 As you can see, it tells me the invoice cannot be negative or greater than 99,999,99.99 - which it clearly isn't.

Hope you can help!

Thanks

Chris