OpenKeyval.org is a completely open key-value data store, exposed as a drop-dead simple web service. The goal is to make this a very easy way to persist data in web applications.
Ran into the same thing; this was a bit frustrating to debug. If the documentation could just be updated that would be awesome. (Also, the link to your GitHub repo is no longer correct).
the documentation states that keys are limited to 128 characters but in fact http://api.openkeyval.org/store/ is refusing keys greater than 64 chars.