Cannot Create Similar User List: USER_LIST_MUTATE_NOT_SUPPORTED

I am attempting to allow users to create a SimilarUserList to a list they already have.  I send a mutate ADD operation with the seedListId, and receive [UserListError.USER_LIST_MUTATE_NOT_SUPPORTED @ operations[0].operand].

Does this mean that one cannot programmatically create a SimilarUserList?  The docs make it seem like you can as it says the seedListId is required when using operator ADD:

seedUserListId

Filterable ReadOnly Required Selectable xsd:long Seed UserListId from which this list is derived. This field can be selected using the value "SeedUserListId". This field can be filtered on. This field is read only and will be ignored when sent to the API for the following Operators: SET. This field is required and should not be null when it is contained within Operators : ADD.

Any thoughts?

Thanks!

--Ralph

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