diff has NUL instead of /dev/null

Hi all.

I am using CVS 1.11.9 which came with Cygwin.

I'm not sure if the version of CVS changed, but
when I was doing an rdiff, I used to (in 2011) get:

Index: hercules/README.S380
diff -c /dev/null hercules/README.S380:1.13
*** /dev/null    Mon Dec 12 18:20:30 2011
--- hercules/README.S380    Thu Mar 11 00:34:48 2010
***************
*** 0 ****
--- 1,43 ----

(note the /dev/null above)

But now I am getting:

Index: hercules/README.S380
diff -c nul hercules/README.S380:1.16
*** nul    Tue Feb 25 02:59:38 2020
--- hercules/README.S380    Wed Oct 02 09:43:01 2019
***************
*** 0 ****
--- 1,125 ----

(note the "nul" above)


I don't think remote "patch" programs can cope
with the word "nul" instead of "/dev/null", and
instead complain that no file called "nul" can be
found.

Who is misbehaving?

Thanks. Paul.