Unable to set an NFeed User property via Rest

[Connect]

My NFeed field has "admin" as a choice and it is selectable in the UI.  Via, Rest, the below Json doesn't seem to work, although it doesn't error either ( 10202 is my NFeed User field )

"update": {},
"fields": {
   ...
   "customfield_10202": [
     {
        "value": "<my email address>"
     }
  ]
}

This actually clears the NFeed field.  Using the user name and key instead of the email address have the same result.