Index: src/wp-includes/link-template.php
===================================================================
--- src/wp-includes/link-template.php	(revision 25333)
+++ src/wp-includes/link-template.php	(working copy)
@@ -2298,7 +2298,7 @@
 
 	if ( 'relative' == $scheme ) {
 		$url = ltrim( preg_replace( '#^\w+://[^/]*#', '', $url ) );
-		if ( $url[0] === '/' )
+		if ( '' !== $url && $url[0] === '/' )
 			$url = '/' . ltrim($url , "/ \t\n\r\0\x0B" );
 	} else {
 		$url = preg_replace( '#^\w+://#', $scheme . '://', $url );