core/modules/comment/src/Entity/Comment.php                          | 1 +
 .../Functional/EntityResource/Comment/CommentResourceTestBase.php    | 5 +++++
 2 files changed, 6 insertions(+)

diff --git a/core/modules/comment/src/Entity/Comment.php b/core/modules/comment/src/Entity/Comment.php
index a244163..dc885fc 100644
--- a/core/modules/comment/src/Entity/Comment.php
+++ b/core/modules/comment/src/Entity/Comment.php
@@ -302,6 +302,7 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
     $fields['entity_type'] = BaseFieldDefinition::create('string')
       ->setLabel(t('Entity type'))
       ->setDescription(t('The entity type to which this comment is attached.'))
+      ->setRequired(TRUE)
       ->setSetting('is_ascii', TRUE)
       ->setSetting('max_length', EntityTypeInterface::ID_MAX_LENGTH);
 
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/Comment/CommentResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/Comment/CommentResourceTestBase.php
index 08816ad..0c99acc 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/Comment/CommentResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Comment/CommentResourceTestBase.php
@@ -280,6 +280,11 @@ public function testPostDxWithoutCriticalBaseFields() {
     $response = $this->request('POST', $url, $request_options);
     $this->assertResourceErrorResponse(422, "Unprocessable Entity: validation failed.\nentity_type: This value should not be null.\n", $response);
 
+    // DX: 422 when invalid 'entity_type' field.
+    $request_options[RequestOptions::BODY] = $this->serializer->encode(['entity_type' => ['value' => 'foobar']] + $this->getNormalizedPostEntity(), static::$format);
+    $response = $this->request('POST', $url, $request_options);
+    $this->assertResourceErrorResponse(422, "Unprocessable Entity: validation failed.\nentity_type: [error message TBD].\n", $response);
+
     // DX: 422 when missing 'entity_id' field.
     $request_options[RequestOptions::BODY] = $this->serializer->encode(array_diff_key($this->getNormalizedPostEntity(), ['entity_id' => TRUE]), static::$format);
     $response = $this->request('POST', $url, $request_options);