"No route found for the specified format API_JSON."

I've installed the latest jsonapi code (from git) against the latest 8.3.x core code (from git). I've enabled the module and cleared all caches. When I load a path like "/jsonapi/node/article/1?_format=API_JSON" I get this error:

$ curl -I -X "GET" "http://example.com/jsonapi/node/article/1?_format=API_JSON"
HTTP/1.1 406 Not Acceptable
Date: Thu, 12 Jan 2017 17:16:36 GMT
Server: Apache/2.2.29 (Unix) mod_ssl/2.2.29 OpenSSL/0.9.8zh PHP/5.6.10
X-Content-Type-Options: nosniff
X-Powered-By: PHP/5.6.10
Cache-Control: must-revalidate, no-cache, private
X-UA-Compatible: IE=edge
Content-language: en
X-Content-Type-Options: nosniff
X-Frame-Options: SAMEORIGIN
Expires: Sun, 19 Nov 1978 05:00:00 GMT
X-Generator: Drupal 8 (https://www.drupal.org)
Content-Length: 49
Content-Type: text/plain; charset=UTF-8

$ curl -X "GET" "http://example.com/jsonapi/node/article/1?_format=API_JSON"
No route found for the specified format API_JSON.

Any ideas?