Eli Zaretskii wrote:
>> >What sort of error?
> "Wrong type argument: char-or-string-p, nil".  See the attached image,

Although you applied the patch to emacs-25, the patch was intended for master. 
emacs-25 and master disagree here because of emacs-25 commit 
2dd3581b78a5fe15e0562b1cc2daa6e3afeeec4a dated December 25 "Restore info about 
the build host in bug reports". This explains why I did not observe the problem 
on master, whereas you observed it on emacs-25.

I just now installed into emacs-25 the attached, which should future-proof the 
Dec. 25 change whenever it happens to be merged into master.

> the reproducible build is not (yet) the standard
> build, it's more like an anomaly at this time, so I thought we should
> have the string explain itself.  And nil cannot explain anything.

That is what documentation is for, no? :-)

Having now had some experience with using the deterministic-build API in code, I 
far prefer using (if FOO ...) to something like (if (string-equal FOO "(elided 
due to deterministic builds)") ...). Not only would the the latter be longer and 
harder to read, it would mean I need to go to the documentation and see how the 
magic-cookie string is spelled and would be likely to make a typo there, which 
means we would need to give the magic cookie a name like 
‘deterministic-build-system-name’ to catch typos so that the code will instead 
be (if (string-equal FOO deterministic-build-system-name) ...), and then we 
would need to give similar names to the magic cookies used for other things like 
emacs-build-time that are elided. This would all be complete overkill. It is 
much simpler to use nil to represent missing information; this is easier to 
remember and read and document and it fits squarely within the Emacs Lisp tradition.