Invalid input data - Aggregate snap

I am building a pipeline that joins some data and then aggregate downstream. However, one of the fields that I am grouping by contains nulls and it gives me this error: Invalid input data.

Failure: Invalid input data, Reason: Missing field in the input data at $[‘Region’], Resolution: Please make sure input data have valid value at $[‘Region’]

If I add this in a mapper before the aggregate, then it will work but does not give me the results I need since I want to aggregate on nulls.

match $[‘Region’]
{ null => “”, _ =>$[‘Region’] }

How do I solve this? Thanks!