diff --git a/Doc/c-api/buffer.rst b/Doc/c-api/buffer.rst
index 230c6a9..9cfd152 100644
--- a/Doc/c-api/buffer.rst
+++ b/Doc/c-api/buffer.rst
@@ -198,6 +198,9 @@
       indicates that no de-referencing should occur (striding in a contiguous
       memory block).
 
+      If all suboffsets are negative (i.e. no de-referencing is needed, then
+      this field must be NULL (the default value). 
+
       This type of array representation is used by the Python Imaging Library
       (PIL). See `complex arrays`_ for further information how to access elements
       of such an array.