diff --git a/src/TagField.php b/src/TagField.php
index 3968c3f..986ed4e 100644
--- a/src/TagField.php
+++ b/src/TagField.php
@@ -312,6 +312,21 @@ class TagField extends DropdownField
         return parent::setValue(array_filter($value));
     }
 
+    /**
+     * Gets the source array if required
+     *
+     * Note: this is expensive for a SS_List
+     *
+     * @return array
+     */
+    public function getSource()
+    {
+        if (is_null($this->source)) {
+            $this->setSource($this->getSourceList());
+        }
+        return $this->source;
+    }
+
     /**
      * {@inheritdoc}
      */