cfssl authsign - "json: cannot unmarshal object into Go value of type []uint8"

I'm trying to use the authsign endpoint with the standard authenticator and I get the following error:

curl -X POST -d '{ "token": "0123456789ABCDEF0123456789ABCDEF", "request":{"certificate_request": "-----BEGIN CERTIFICATE REQUEST-----\nMIIBUjCB+QIBADBqMQswCQYDVQQGEwJVUzEUMBIGA1UEChMLZXhhbXBsZS5jb20x\nFjAUBgNVBAcTDVNhbiBGcmFuY2lzY28xEzARBgNVBAgTCkNhbGlmb3JuaWExGDAW\nBgNVBAMTD3d3dy5leGFtcGxlLmNvbTBZMBMGByqGSM49AgEGCCqGSM49AwEHA0IA\nBK/CtZaQ4VliKE+DLIVGLwtSxJgtUKRzGvN1EwI3HRgKDQ3l3urBIzHtUcdMq6HZ\nb8jX0O9fXYUOf4XWggrLk1agLTArBgkqhkiG9w0BCQ4xHjAcMBoGA1UdEQQTMBGC\nD3d3dy5leGFtcGxlLmNvbTAKBggqhkjOPQQDAgNIADBFAiAcvfhXnsLtzep2sKSa\n36W7G9PRbHh8zVGlw3Hph8jR1QIhAKfrgplKwXcUctU5grjQ8KXkJV8RxQUo5KKs\ngFnXYtkb\n-----END CERTIFICATE REQUEST-----"}}' http://localhost:8888/api/v1/cfssl/authsign | python -m json.tool

{
"errors": [
{
"code": 400,
"message": "json: cannot unmarshal object into Go value of type []uint8"
}
],
"messages": [],
"result": null,
"success": false
}

My cfssl config file:
{
"signing": {
"profiles": {
"CA": {
"usages": ["cert sign"],
"expiry": "720h",
"auth_key": "ca-auth"
},
"email": {
"usages": ["s/mime"],
"expiry": "720h"
}
},
"default": {
"usages": ["signing", "key encipherment", "server auth", "client auth"],
"expiry": "8760h",
"auth_key": "ca-auth"
}
},
"auth_keys": {
"ca-auth": {
"type":"standard",
"key":"0123456789ABCDEF0123456789ABCDEF"
}
},
"remotes": {
"localhost": "0.0.0.0:8888"
}
}