diff --git a/doc/api/resources/orders.rst b/doc/api/resources/orders.rst
index 429fdb58b..1282dc3ec 100644
--- a/doc/api/resources/orders.rst
+++ b/doc/api/resources/orders.rst
@@ -853,7 +853,7 @@ Creating orders
 
    You can supply the following fields of the resource:
 
-   * ``code`` (optional)
+   * ``code`` (optional) – Only ``A-Z`` and ``0-9``, but without ``O`` and ``1``.
    * ``status`` (optional) – Defaults to pending for non-free orders and paid for free orders. You can only set this to
      ``"n"`` for pending or ``"p"`` for paid. We will create a payment object for this order either in state ``created``
      or in state ``confirmed``, depending on this value. If you create a paid order, the ``order_paid`` signal will