rng_count not working as expected with CombinationProxy objects

When I try to use the rng_count method defined in treefunctions.py on objects returned by the combine method, I found an unexpected error:

 File "/path/to/my/bamboovenv/lib/python3.7/site-packages/bamboo/treeoperations.py", line 1078, in fromRngFun
    accuExpr = adaptArg(accuFun(prevRes.result, rng._getItem(idx.result)))
AttributeError: 'CombinationProxy' object has no attribute '_getItem'

Here is a pseudocode version of what I expected to work was defining a set of objects:

jet_pairs = combine( jet_collection, N=2)
special_pair = rng_max_element_by( jet_pairs, lambda pair : jet_pair_specialness(pair[0], pair[1]) )

and then use the rng_count method, for example as an expression when plotting , with an expression such as:

expr = rng_count(special_pair, lambda jet : is_super_extra_special(jet) )

but this gives the above error. The same occurs if the rng_count function is called with no lambda specified. It would be nice to be able to do these kinds of operations.