Right now, not all allowed aggregations are invariant (meaning that the members of the group do not change over time). However, the collector object requires invariant aggregations. Using an aggregation that isn't invariant will cause the simulation to fail.
source:/trunk/source/core/aggregate.c
This is probably ok as is. This might be repaired using a global flag that is more permissive but would result in much slower simulations.
Opinions sought.