[Exim] ${sender_address} returning blank

I'm very stumped on this one, and am HOPING that someone on this list can
help me out.

I have a transport in my exim.conf file that passes ${sender_address} on a
command line. Now I've read the Dox and from what I can see, a message FROM
ABC@??? TO XYZ@??? should resolve like this

${sender_address} = ABC@???
${pipe_address} = XYZ@???

right? However, when I run exim in debug mode and watch it passing the
variables out, I get this:

argv[1] = -f
argv[2] = ${sender_address}
argv[3] = -t
argv[4] = ${pipe_address}
direct command:
argv[0] = /usr/sbin/amavis
argv[1] = -f
argv[2] = ${sender_address}
argv[3] = -d
argv[4] = ${pipe_addresses}
direct command after expansion:
argv[1] = -f
argv[2] =
argv[3] = -d
argv[4] = ABC@???

Now am I totally lost or is that quite wrong?