Creating a cost assignment

Hi,
I've been working this days on creating cost assignments from PSI. I've found something strange in Project Server behaviour. When you create a new AssignmentRow and you select RES_TYPE = 25 (cost resource) and try to update the project, Project Server automaticaly cancell the job in the queue. I don't know why Project Server does that and the only workarround I found (and I know that is not the better workarround) was create costs assignments as work assignments and indicate in assignment Group that resource is from cost Group. If you do this there is another problem...You can't assign a cost to the resource (ASSN_COST) becouse is not a cost resource, so you have to work with ASSN_UNITS field. The problem here is that if you modify resource's units and TASK_TYPE is in mode Fixed Units (as default) will modify task duration too, so you need to set the TASK_TYPE as Fixed Duration before you assign the resource.