Access _POST data in plugin

I am writing a plugin that has a form, receives and writes data to a custom database table and sends an email.
In my form I use <input type="hidden" name="action" value="peoples_choice_vote"> and my function is called like this

if ( is_this_user_logged_in() ) {
    add_action('admin_post_peoples_choice_vote', 'handle_peoples_choice_vote');
} else {
    add_action('admin_post_nopriv_peoples_choice_vote', 'handle_peoples_choice_vote');
}

This code works fine in functions.php when I move it to the plugin and activate it fails, and when I submit the form I get a white screen at admin-post.php, no data is written and the email is not sent.