Index: src/wp-includes/class-wp-list-util.php
===================================================================
--- src/wp-includes/class-wp-list-util.php	(revision 53756)
+++ src/wp-includes/class-wp-list-util.php	(working copy)
@@ -164,9 +164,9 @@
 			 */
 			foreach ( $this->output as $key => $value ) {
 				if ( is_object( $value ) ) {
-					$newlist[ $key ] = $value->$field;
+					$newlist[ $key ] = isset( $value->$field ) ? $value->$field : $value;
 				} else {
-					$newlist[ $key ] = $value[ $field ];
+					$newlist[ $key ] = isset( $value[ $field ] ) ? $value[ $field ] : $value;
 				}
 			}
 
@@ -182,15 +182,15 @@
 		foreach ( $this->output as $value ) {
 			if ( is_object( $value ) ) {
 				if ( isset( $value->$index_key ) ) {
-					$newlist[ $value->$index_key ] = $value->$field;
+					$newlist[ $value->$index_key ] = isset( $value->$field ) ? $value->$field : $value;
 				} else {
-					$newlist[] = $value->$field;
+					$newlist[] = isset( $value->$field ) ? $value->$field : $value;
 				}
 			} else {
 				if ( isset( $value[ $index_key ] ) ) {
-					$newlist[ $value[ $index_key ] ] = $value[ $field ];
+					$newlist[ $value[ $index_key ] ] = isset( $value[ $field ] ) ? $value[ $field ] : $value;
 				} else {
-					$newlist[] = $value[ $field ];
+					$newlist[] = isset( $value[ $field ] ) ? $value[ $field ] : $value;
 				}
 			}
 		}