I'm trying to update a supporter record and running in to a 500 error.

I did a PUT request to
https://[nation].nationbuilder.com/api/v1/people/[id]?access_token=[token]
using the data:
{"person":{"first_name":"Jane","last_name":"Smith","phone":"123-456-7890"}}

This is the response:
HTTP/1.1 500 Internal Server Error
Date: Wed, 09 Apr 2014 14:38:59 GMT
Server: Apache/2.2.22 (Ubuntu)
X-UA-Compatible: IE=Edge,chrome=1
Cache-Control: no-cache
Nation-RateLimit-Limit: 15000
Nation-RateLimit-Remaining: 14978
Nation-RateLimit-Reset: 1397088000
X-Request-Id: 61ca028a58830a98d0c83ce60c18d079
X-Runtime: 0.532239
X-Rack-Cache: invalidate, pass
X-Powered-By: Phusion Passenger 4.0.23
Status: 500 Internal Server Error
Vary: Accept-Encoding
Content-Type: application/json
Via: 1.1 nationbuilder.com
Connection: close
Transfer-Encoding: chunked
{"code":"server_error","message":"You have encountered a server error.","inner_error":{"name":"NoMethodError","message":"undefined method `strip' for nil:NilClass"}}

The same data POSTed to the create endpoint successfully created a supporter, and I'm using the same function to PUT tags.

Any ideas what could be causing this?