diff --git a/src/wp-includes/class-wp-comment-query.php b/src/wp-includes/class-wp-comment-query.php
index c8fed2586a..f8d716ce79 100644
--- a/src/wp-includes/class-wp-comment-query.php
+++ b/src/wp-includes/class-wp-comment-query.php
@@ -1013,6 +1013,9 @@ class WP_Comment_Query {
 		// Prime comment caches for non-top-level comments.
 		$descendant_ids = array();
 		for ( $i = 1, $c = count( $levels ); $i <= $c; $i++ ) {
+			if ( empty( $levels[ $i ] ) ) {
+				continue;
+			}
 			$descendant_ids = array_merge( $descendant_ids, $levels[ $i ] );
 		}