diff --git a/Domain/CustomAttribute.php b/Domain/CustomAttribute.php
index ea67a8543..62d1bd97a 100644
--- a/Domain/CustomAttribute.php
+++ b/Domain/CustomAttribute.php
@@ -76,7 +76,7 @@ class CustomAttribute
     protected $adminOnly = false;
 
     /**
-     * @var string
+     * @var string|null
      */
     protected $possibleValues;
 
@@ -134,6 +134,9 @@ public function PossibleValues()
      */
     public function PossibleValueList()
     {
+        if (is_null($this->possibleValues)) {
+            return [];
+        }
         return explode(',', $this->possibleValues);
     }