diff --git a/wp-includes/rewrite.php b/wp-includes/rewrite.php
index ab23a1887c..af66d02b94 100644
--- a/wp-includes/rewrite.php
+++ b/wp-includes/rewrite.php
@@ -410,7 +410,10 @@ function wp_resolve_numeric_slug_conflicts( $query_vars = array() ) {
 	}
 
 	// This is the potentially clashing slug.
-	$value = $query_vars[ $compare ];
+	$value = '';
+	if ( $compare && array_key_exists( $compare, $query_vars ) ) {
+		$value = $query_vars[ $compare ];
+	}
 
 	$post = get_page_by_path( $value, OBJECT, 'post' );
 	if ( ! ( $post instanceof WP_Post ) ) {
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 4587006415..8ac87d7b55 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53856';
+$wp_version = '6.1-alpha-53857';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.