Tuum can be accusative, or nominative, because "munus, -eris" is a neutral noun. That means that "munus tuum" is linked to the object or to the subject of "captus".
As there isn't the context, I cannot be sure, so I give the two options. But, in my opinion, the second is the correct, as I think this phrase come from a text in first person.
Τελευταία επικύρωση ή επεξεργασία από Taty - 6 Σεπτέμβριος 2006 15:28
Thanks Mistaya, when you have several translation alternative, only keep the best one in the main field and the others in the comments. I've edited the translation.