diff --git src/wp-includes/theme.php src/wp-includes/theme.php
index 49ecdbb..8979886 100644
--- src/wp-includes/theme.php
+++ src/wp-includes/theme.php
@@ -1561,6 +1561,7 @@ function add_theme_support( $feature ) {
 
 		case 'custom-logo':
 			$defaults = array(
+				'size'        => '',
 				'width'       => null,
 				'height'      => null,
 				'flex-width'  => false,