WordPress 5 / Gutenberg – theme doesn’t have featured image option showing up

I have a custom theme. I’ve deactivated every plugin. I can’t get the Featured Image block on the right sidebar of editing a post to show up so that the user can set the featured image.

I have this in my functions.php file:

add_theme_support( 'post-thumbnails' );

Note that this worked fine before WordPress 5 / Gutenberg update.

What am I missing?

The function with the add_theme_support( ‘post-thumbnails’ ); wasn’t being called for some reason. Once I fixed that, everything works now.