Issue #19795: Mark up None as literal text.
diff --git a/Doc/library/bdb.rst b/Doc/library/bdb.rst
index 8ee9921..116ffcf 100644
--- a/Doc/library/bdb.rst
+++ b/Doc/library/bdb.rst
@@ -241,7 +241,7 @@
    .. method:: set_continue()
 
       Stop only at breakpoints or when finished.  If there are no breakpoints,
-      set the system trace function to None.
+      set the system trace function to ``None``.
 
    .. method:: set_quit()