diff --git a/library/Director/PropertyModifier/PropertyModifierRejectOrSelect.php b/library/Director/PropertyModifier/PropertyModifierRejectOrSelect.php
index 04c49c57f..0a18ec8a9 100644
--- a/library/Director/PropertyModifier/PropertyModifierRejectOrSelect.php
+++ b/library/Director/PropertyModifier/PropertyModifierRejectOrSelect.php
@@ -77,7 +77,10 @@ public static function addSettingsFormFields(QuickForm $form)
 
     public function matchesRegexp($string, $expression)
     {
-        return preg_match($expression, $string);
+        if ($string === null) {
+            $string = '';
+        }
+        return preg_match($expression, (string) $string);
     }
 
     public function isNull($string, $expression)