[m-rev.] diff: fix a VERY old bug

[Julien Fischer]

On Sun, 16 Nov 2014, Zoltan Somogyi wrote:

> The bug that this diff fixes has been there since Peter Ross
> added the buggy macro to the file in 2002! We hadn't noticed
> it because the call to the macro in the library passed Str
> as the string argument, so the code *accidentally* worked
> in this case.
>
> Julien, please apply this to the release branch if you are
> still maintaining that branch.

Shall do.

Cheers,
Julien.