EXCEPTION halt in \halopc\haloce\source\tag_files\tag_groups.c,#3157: #-1 is not a valid animation_graph_unit_seat_block index in [#0,#1)

This is what OS_Sapien tells me when I try to open a50. I had no problems with the old version of Open Sauce.

Thanks to this thread (http://forum.halomaps.org/index.cfm%3Fpage=topic&topicID=45308) I know that the error should be caused by something messed up with the animating functions in the model_animation tag. This, however, doesn't seem to be the case: the problem rise with Sapien, but Tool compiles just fine, and the map in-game works with no problems. Moreover, I opened the compiled map with HEK+ to trace each and every model_animation tag used by the scenario, checked all of them one by one, and none of them seem to have functions at all, except "vehicles\c gun turret\c gun turret". Now I'd like to know: is there a way to know exactly which tag is causing this? And may I know if there is a fix?