Traceback NA's to their first appearance

Hello,

I've written a relatively complex interconnected population model in R. 
When changing a certain parameter, the outputs end up containing NA's.
I would like to find out, in which step the model (in form of a loop) 
starts to produce NA's. Does anyone know how to achieve this?
Since it does not give an error I can't use the traceback() function.

Thank you very much in advance
Ben