ivpoisson and multiplicative/additive erros

[Michael Andersonnnn]

Hi, I was wondering if anyone understood why ivpoisson only runs with multiplicative errors when analysing length of stay?
When I try to run with additive errors, and I do when using xtpoisson I get the following error term:

irr not appropriate under additive errors

My code is as follows:

(additive errors)

xi: ivpoisson gmm postoplos (ind=diff1) i.agecat sex i.imddecile i.charlindex i.volq yr1 yr2 yr3 yr4 weekdays winter, irr

(multiplicative errors)

xi: ivpoisson gmm postoplos (ind=diff1) i.agecat sex i.imddecile i.charlindex i.volq yr1 yr2 yr3 yr4 weekdays winter, irr multiplicative