Custom policies and System Architect integration

Hi, using the new System Architect integration I wrote a custom policy to create a description based in informatioin extracted from the xml file. The policy works if I save the xml file first and submit it without using the integration but it fails if I submit it via the integration - I get error: (500) Internal Server Error.

I got round it by shifting my policy from the initial (Submitted) state to the Approved state, so the description is actually added on that transition, but is there any reason why the original version should not work?