diff --git a/core/modules/rest/src/ResourceResponse.php b/core/modules/rest/src/ResourceResponse.php
index eb955c293c28b52cac77c5cc252dec98810dfbb4..e6900ec2449b5ae87b7e9f7d93ccb6e5b1a409f9 100644
--- a/core/modules/rest/src/ResourceResponse.php
+++ b/core/modules/rest/src/ResourceResponse.php
@@ -14,6 +14,8 @@
  * string or an object with a __toString() method, which is not a requirement
  * for data used here.
  *
+ * Routes that return this response must specify the '_format' requirement.
+ *
  * @see \Drupal\rest\ModifiedResourceResponse
  */
 class ResourceResponse extends Response implements CacheableResponseInterface, ResourceResponseInterface {