diff --git a/unique_field.module b/unique_field.module
index 804e79d..9224b7b 100644
--- a/unique_field.module
+++ b/unique_field.module
@@ -332,7 +332,7 @@ function unique_field_form_validate(array &$form, FormStateInterface $form_state
         if ($field_type == 'link') {
           $field_values[] = $value['uri'];
         }
-        else {
+        else if (is_array($value)) {
           $field_values[] = ($field_type == 'entity_reference') ? $value['target_id'] : $value['value'];
         }
       }