I am trying to run a multi-row formula in a workflow, and I keep getting this error:
The field "" is missing. Compare the tool configuration with the input stream.

Here is my expression:

The only field I'm referencing in the expression is "Field1," and this field is clearly in the workflow, as you can see in the "Group By" option. Other posts on similar errors have suggested looking for "implied" fields in the metadata, but I can't find any. There are also no blank spaces or returns in my expression.
What am I doing wrong?
Thanks!