diff --git a/src/wp-includes/vars.php b/src/wp-includes/vars.php
index 5f9257e85a..e285ed8321 100644
--- a/src/wp-includes/vars.php
+++ b/src/wp-includes/vars.php
@@ -29,9 +29,11 @@ if ( is_admin() ) {
 	} else {
 		preg_match( '#/wp-admin/?(.*?)$#i', $_SERVER['PHP_SELF'], $self_matches );
 	}
-	$pagenow = $self_matches[1];
+
+	$pagenow = ! empty( $self_matches[1] ) ? $self_matches[1] : '';
 	$pagenow = trim( $pagenow, '/' );
 	$pagenow = preg_replace( '#\?.*?$#', '', $pagenow );
+
 	if ( '' === $pagenow || 'index' === $pagenow || 'index.php' === $pagenow ) {
 		$pagenow = 'index.php';
 	} else {