My AMP page validates, but on load has an uncaught error in the JavaScript console saying:

Uncaught Error: form action attribute is invalid for method=POST: form#register​​​
    at xa.f.assert (https://cdn.ampproject.org/v0.js:11:342)
    at HTMLDocument.Hf (https://cdn.ampproject.org/v0.js:164:50)

I don't undertand the error since the forms action could be used in method=POST (as far I did for years in HTML)

How do we reproduce the issue?

When visiting http://iberopistacho.com/registro this error should appear on load in the Javascript console.

I'm running ChromeOS Version 54.0.2840.59 (64-bit)
Powered by AMP ⚡ HTML – Version 1477334765771
and using https://cdn.ampproject.org/v0.js