“Bad hCaptcha nonce!” on arbitrary form

I’m working on a form of my own and I believe I have everything correctly configured. When filled in, the form validates and posts. I can see the field data server-side, including h-captcha-response. The wordpress plugin is installed, activated, correctly linked and validates (via the tool in settings).

But alas, the hcaptcha_verify_post() function refuses to return anything but “Bad hCaptcha nonce!“. Any clues as to what I might be missing are much appreciated.

The page posts to itself. This isn’t finished but here’s the server side code as it currently stands:

if( isset( $_POST ) && $_SERVER['REQUEST_METHOD'] === 'POST') {
	debug("we were POSTed to...");
	$result = hcaptcha_verify_post();
	if( null !== $result){
		debug(esc_html($result));
		debug("invalid captcha");

	}else{
		debug("captcha is valid");
	}
	foreach ($_POST as $key => $value) {
		//Remove all slashes, php and html tags, and quote html entities.
		$_POST[$key] = htmlentities(strip_tags(removeslashes($value)), ENT_QUOTES, "UTF-8");
	}
	debug(print_r($_POST,true));
}

The page I need help with: [log in to see the link]