Ticket #61408: 61408.2.diff

src/wp-includes/general-template.php

@@ -1035 +1035 @@
         }
 
         $custom_logo_id = get_theme_mod( 'custom_logo' );
-        $is_image       = wp_attachment_is_image( $custom_logo_id );
+        $is_image       = ( $custom_logo_id ) ? wp_attachment_is_image( $custom_logo_id ) : false;
 
         if ( $switched_blog ) {
                 restore_current_blog();