If get_field Equals False

I’m trying to replace an image with a gravatar if the image in the custom field returns false(or does not exisit)

So I’ve set my variable

<?php $thumb_image = get_field('thumb_image'); ?>

What is the value if there is no image in the custom field thumb_image? This is not working

<?php if ($thumb_image == false) : ?>
 <?php echo get_avatar( get_the_author_meta('ID') ); ?>
<?php endif; ?>

http://wordpress.org/extend/plugins/advanced-custom-fields/