post-formats.php fuctions all undefined

I installed 3.6 and ran into this error:

Fatal error: Call to undefined function get_post_format_slugs() in /home/ … /wp-includes/theme.php on line 1264

After a fresh download and reinstall, the error persisted so I investigated.

get_post_format_slugs() is defined in wp-includes/post-formats.php on line 115 as follows:

function get_post_format_slugs() {
	$slugs = array_keys( get_post_format_strings() );
	return array_combine( $slugs, $slugs );
}

So in theme.php I edited the call to get_post_format_slugs() to avoid calling the function and just use get_post_format_strings() as it does above to see if that would help, but only to receive the same error again, but this time about the get_post_format_string() function.

get_post_format_strings() is indeed defined — it’s in post-formats.php on line 90.

What’s up here? It seems like my post-formats.php isn’t being included, or something.