diff --git a/library/Director/Objects/IcingaTimePeriodRanges.php b/library/Director/Objects/IcingaTimePeriodRanges.php
index 7e17b7f3..345c0afb 100644
--- a/library/Director/Objects/IcingaTimePeriodRanges.php
+++ b/library/Director/Objects/IcingaTimePeriodRanges.php
@@ -101,6 +101,7 @@ class IcingaTimePeriodRanges implements Iterator, Countable, IcingaConfigRendere
                     'timeperiod_key'      => $range,
                     'timeperiod_value'    => $value,
                 ));
+                $this->modified = true;
             }
         }