diff --git a/core/modules/link/src/Tests/LinkFieldTest.php b/core/modules/link/src/Tests/LinkFieldTest.php
index 0b034d1..4eeaf56 100644
--- a/core/modules/link/src/Tests/LinkFieldTest.php
+++ b/core/modules/link/src/Tests/LinkFieldTest.php
@@ -163,6 +163,8 @@ public function testURLValidation() {
     // Test external URLs for 'link_type' = LinkItemInterface::LINK_EXTERNAL.
     $this->field->setSetting('link_type', LinkItemInterface::LINK_EXTERNAL);
     $this->field->save();
+    // Missing protocol schema.
+    $invalid_external_entries['www.drupal.org'] = $validation_error_1;
     $this->assertValidEntries($field_name, $valid_external_entries);
     $this->assertInvalidEntries($field_name, $valid_internal_entries + $invalid_external_entries);