I managed to run the query without getting the error but it still seems like there is some strange behaviour going on. I pass all the tests except for the last random one (Find Inventory Changes (Randomized Data) validates the query against multiple random date combinations.

In general, I think you were on the right track with the nulls - returning a NULL generates the initial error.

Sounds good and thanks for the quick response. As far as I can tell, nulls are not being returned but I'll give it another look in a little bit. To add some info, this what comes back from the backtrace:
./spec/query_spec.rb:168:in map' ./spec/query_spec.rb:168:in format_output'
./spec/query_spec.rb:158:in block (3 levels) in <top (required)>' ./spec/query_spec.rb:127:in times'
./spec/query_spec.rb:127:in block (2 levels) in <top (required)>' ./spec/query_spec.rb:6:in block (3 levels) in <top (required)>'
/usr/local/bundle/gems/sequel-5.47.0/lib/sequel/database/transactions.rb:251:in _transaction' /usr/local/bundle/gems/sequel-5.47.0/lib/sequel/database/transactions.rb:233:in block in transaction'
/usr/local/bundle/gems/sequel-5.47.0/lib/sequel/connection_pool/threaded.rb:92:in hold' /usr/local/bundle/gems/sequel-5.47.0/lib/sequel/database/connecting.rb:269:in synchronize'
/usr/local/bundle/gems/sequel-5.47.0/lib/sequel/database/transactions.rb:195:in transaction' ./spec/query_spec.rb:5:in block (2 levels) in <top (required)>'

I can't complete as I'm getting this error: ./spec/query_spec.rb:169:in block in format_output': undefined method ljust' for nil:NilClass (NoMethodError)
Any idea what is going on?