How to use "Group By" block

[carrrson]

Hi!
I have been playing around with flows and am stumped on how to use the group by block. I am passing in a list and hoping to group the objects by the county property, but keep getting "QueryError: Property is not a complex type" when I try to run the block. I've tried passing in the default and body values as well, but can't seem to find a configuration that works. Very possible I'm misunderstanding the purpose/use of the block, but grateful for any insight. Thanks!

Here is the url if anyone wants to play around with it (no auth required): https://data.ny.gov/resource/qq4h-8p86.json

my current setup:

for reference, here is my data structure