Orderby => meta_value Not Working

I have a query, and am trying to sort via a custom field but it’s not working. It doesn’t change / sort anything at all.

Replacing this with orderby => title works fine, so I know it’s not a sorting issue elsewhere in the code…

Here’s a complete summary of my code, I’m really stumped and hoping someone might have a word of advice. I’m using the “Types” plugin, hence the reference to “types_render_field…”
Thanks so much in Advance!

<?php
	// Set arguments for the query in a php array.
	$args = array(
		'posts_per_page' => 100,
		'post_type' => 'workshop-date',
		'meta_key' => types_render_field("date", array("output"=>"raw")),
		'orderby' => 'meta_value',
		'order' => 'ASC');

	$query = new wp_query( $args );

	//While you still have posts (haven't displayed the last one) - then display it!
	if ( $query -> have_posts() ) : while ( $query -> have_posts() ) : $query -> the_post();

?>

 //HTML Output of posts here

<?php

	endwhile;
	else:
?>

		<p>There are no dates to display.</p>

<?php endif; ?>