Identifier REST API Add returns 500 instead of 403 for existing identifier

When adding an Identifier that already exists the REST API returns a 500 instead of the documented 403.

Here is an example Add/POST:

(
    [method] => POST
    [uri] => Array
        (
            [scheme] => http
            [host] => 127.0.0.1
            [user] => co_2.rest_user_01
            [pass] => XXXXXXXX
            [path] => /registry/identifiers.json
        )

    [header] => Array
        (
            [Content-Type] => application/json
        )

    [body] => {"RequestType":"Identifiers","Version":"1.0","Identifiers":[{"Version":"1.0","Type":"eppn","Identifier":"scott.koranda@ncsa.illinois.edu","Login":true,"Person":

,"Status":"Active"}]}
)

If that identifier value already exists it should return a 403 but returns a 500.