Query vars get me a 404

I’m trying to pass some variables to a page via the URL:

mysite.com/archive-posttype/?key=value

But sometimes I get a 404. Not all the times, just with some values.

I tried using this:

add_filter('query_vars', 'cc_add_my_var');
function cc_add_my_var($public_query_vars) {
	array_push($public_query_vars, 'key1', 'key2', 'key3');
	return $public_query_vars;
}

But nothing. It think that it’s a rewrite issue, but I’m not sure what to do now.