diff --git a/core/modules/link/src/Plugin/Field/FieldWidget/LinkWidget.php b/core/modules/link/src/Plugin/Field/FieldWidget/LinkWidget.php
index e90d416..28d5839 100644
--- a/core/modules/link/src/Plugin/Field/FieldWidget/LinkWidget.php
+++ b/core/modules/link/src/Plugin/Field/FieldWidget/LinkWidget.php
@@ -227,6 +227,11 @@ public function formElement(FieldItemListInterface $items, $delta, array $elemen
     elseif ($this->supportsExternalLinks() && $this->supportsInternalLinks()) {
       $element['uri']['#description'] = $this->t('Start typing the title of a piece of content to select it. You can also enter an internal path such as %add-node or an external URL such as %url. Enter %front to link to the front page.', array('%front' => '<front>', '%add-node' => '/node/add', '%url' => 'http://example.com'));
     }
+    // If the field is configured to allow only external links,
+    // show a useful description.
+    elseif ($this->supportsExternalLinks() && !$this->supportsInternalLinks()) {
+      $element['uri']['#description'] = $this->t('This must be an external URL such as %url.', array('%url' => 'http://example.com'));
+    }
 
     $element['title'] = array(
       '#type' => 'textfield',