Issue with $union Operation in Mixpanel Profile Batch Update API

Hey. We had a number of user profiles created without the association to our 'company' Group. The 'company' value (which is the Group key) on the User profile is a List type. I am trying to use the Profile Batch Update - Engage API. Here is my endpoint: https://api.mixpanel.com/engage?ip=0&verbose=1#profile-batch-update Here is a single object from my batch payload: { "$token": "qwerty123456", "$distinct_id": "1234", "$set": { "$email": "example@example.com" }, "$union": { "company": [ "1234" ] } } I keep getting an error when attempting to send with the $union operation. I feel like I'm doing something wrong here. When I try to update a small batch with just the $set operation, the requests are successful. Any help would be appreciated!