diff --git a/Indextank/Index.php b/Indextank/Index.php
index 5576e54..ffe4449 100644
--- a/Indextank/Index.php
+++ b/Indextank/Index.php
@@ -395,7 +395,7 @@ private function as_search_param( $query, $start = NULL, $len = NULL, $scoring_f
             $params["fetch"] = $fetch_fields;
         }
         if ($category_filters != NULL) {
-            $params["category_filters"] = $category_filters;
+            $params["category_filters"] = json_encode($category_filters);
         }
         if ($variables) {
             foreach ($variables as $k => $v)