get_avatar not working

Hey guys,

Custom theme > single post > inside the loop
I call this
<?php echo get_avatar( get_the_author_meta('user_email'), 144 ); ?>
And it doesn’t work. (I get the mystery man) When I manually insert an email it does work (It show me the correct avatar):
<?php echo get_avatar( '[email protected]', 144 ); ?>

Using this line
<?php echo get_the_author_meta('user_email'); ?>
works fine too.

My settings are correct (show avatars).

Does anybody have any idea what could be causing this?