This will be fixed in the public 1.7. This was one of the bugs that had been bugging me but was a pain to track down because it only happened occasionally, whenever we tried to test for it it worked fine (turns out it was the type of terrain used in the tactical map that would determine when it would and wouldn't work!?!). Most of the time it just showed as the ai casting a summon and nothing happened, but your case could happen as well.
