diff --git a/service.inc b/service.inc
index 51e9202..b5aee02 100644
--- a/service.inc
+++ b/service.inc
@@ -621,7 +621,7 @@ class SearchApiDbService extends SearchApiAbstractService {
           if (is_array($value)) {
             foreach ($value as $v) {
               if ($v !== NULL) {
-                $values[$v] = TRUE;
+                $values["$v"] = TRUE;
               }
             }
             $values = array_keys($values);