CAncelled Tax Line does not allow to cancel an invoice

HI,

I have a strange situation;

An invoice was created, however there seemed to be a tax issue onit and the invoice needed to be cancelled.

Tried to cancel once and the system reverses the item lines and the distribution lines with no issue however in the line tab the tax line still has an amount on it but the distribution field has no value in it.

and the system does not allow us to amend anything at all even via help>diagnostics>examine.

any ideas?

Thanks

S