How to delete clientUserId on a recipient?

Hi, we are working on a DocuSign integration allowing users to sign documents in-app. We do have requirements to allow users to forward the signature to external users. We initially create the Signer record with a clientUserId (so the user can sign in-app using the embedded signature), and to forward the signature, we are trying to update the signer record with the external user info and remove the clientUserId value so Docusign would send out an email inviting them to sign the envelope. However, when updating the Signer, and not specifying the clientUserId (as the documentation mentions), the clientUserId persists. Updating the clientUserId to another value does work, but deleting it doesn’t. Any recommendations here?