diff --git a/Doc/library/random.rst b/Doc/library/random.rst
index ef0cfb0e76cef6..0e20f697b048b6 100644
--- a/Doc/library/random.rst
+++ b/Doc/library/random.rst
@@ -115,6 +115,9 @@ Functions for bytes
 
    Generate *n* random bytes.
 
+   *n* must be greater or equal to ``0``. The upper bound depends
+   on platform integer size.
+
    This method should not be used for generating security tokens.
    Use :func:`secrets.token_bytes` instead.