diff --git a/library/Icinga/Data/Db/DbQuery.php b/library/Icinga/Data/Db/DbQuery.php
index fffd7b08a..04f96ddb8 100644
--- a/library/Icinga/Data/Db/DbQuery.php
+++ b/library/Icinga/Data/Db/DbQuery.php
@@ -251,7 +251,7 @@ class DbQuery extends SimpleQuery
     protected function valueToTimestamp($value)
     {
         // We consider integers as valid timestamps. Does not work for URL params
-        if (ctype_digit($value)) {
+        if (! is_string($value) || ctype_digit($value)) {
             return $value;
         }
         $value = strtotime($value);