Supabase & Google Maps

I'm trying to pull database rows from Supabase to populate Google Map markers. Because Supabase does not have a LatLng type, I'm using a custom function to take strings and parse them into LatLng types.

I got it working, but recently it hasn't, and I'm wondering if something has changed. I'm now getting a red screen with the error:

TypeError: Instance of '_JsonMap': type '_JsonMap' is not a subtype of type 'String'

Which leads me to think that what Flutterflow is getting from Supabase is JSON formatted. But when I try setting my custom code argument to JSON, I cannot select the column because in the schema it's a string.