strange interaction between '[0]' and -define pdf:use-cropbox=true

I have been messing with cropping pdf's, and (subsequently) found that
    -define pdf:use-cropbox=true
was a fine and helpful thing.

Since I also (sometimes) deal with multi page PDF's
my normal thumbnail script includes

${src}'[0]'

Which has worked fine up til now.

But if I use BOTH options, I don't get the result I expect,
even on a single page pdf.

In short:

convert -define pdf:use-cropbox=true v.pdf v.tif

does what I want, and

convert -define pdf:use-cropbox=true v.pdf'[0]' v.tif

doesn't, even though v.pdf happens to be single page.

I'm presently on
Version: ImageMagick 6.3.5 09/20/07 Q16 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2007 ImageMagick Studio LLC

   BugBear