diff --git a/wp-includes/taxonomy.php b/wp-includes/taxonomy.php
index b11ab45acb..010d33f8c8 100644
--- a/wp-includes/taxonomy.php
+++ b/wp-includes/taxonomy.php
@@ -743,7 +743,7 @@ class WP_Tax_Query {
 			}
 
 			$terms = $query['terms'];
-			$operator = $query['operator'];
+			$operator = strtoupper( $query['operator'] );
 
 			if ( 'IN' == $operator ) {
 
@@ -3993,4 +3993,4 @@ function wp_check_term_hierarchy_for_loops( $parent, $term_id, $taxonomy ) {
 		wp_update_term( $loop_member, $taxonomy, array( 'parent' => 0 ) );
 
 	return $parent;
-}
\ No newline at end of file
+}