--- a/core/lib/Drupal/Component/Uuid/Uuid.php
+++ b/core/lib/Drupal/Component/Uuid/Uuid.php
@@ -20,6 +20,9 @@
   /**
    * Checks that a string appears to be in the format of a UUID.
    *
+   * Only lower case strings are considered valid for consistency and
+   * compatibility reasons.
+   *
    * Implementations should not implement validation, since UUIDs should be in
    * a consistent format across all implementations.
    *