diff --git a/docs/library/machine.Pin.rst b/docs/library/machine.Pin.rst
index 5254e163c0..3055491ebb 100644
--- a/docs/library/machine.Pin.rst
+++ b/docs/library/machine.Pin.rst
@@ -216,6 +216,7 @@ Methods
      - ``hard`` if true a hardware interrupt is used. This reduces the delay
        between the pin change and the handler being called. Hard interrupt
        handlers may not allocate memory; see :ref:`isr_rules`.
+       Not all ports support this argument.
 
    This method returns a callback object.