Index: src/wp-includes/general-template.php
===================================================================
--- src/wp-includes/general-template.php	(revision 58374)
+++ src/wp-includes/general-template.php	(working copy)
@@ -1024,7 +1024,7 @@
 	}
 
 	$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();