zero ions matched using X!Tandem + kscore on ETD data

[GATTACA]

Hello All,

I'm using the latest stable release of TPP (4.3.1) to search some
phospho ETD data.
I've modified my tandem input file for ETD:
scoring, x ions: no
scoring, y ions: no
scoring, z ions: yes
scoring, a ions: no
scoring, b ions: no
scoring, c ions: yes

residue, potential modification mass: 15.994915@M,57@C
residue, potential modification motif: 79.99@[STY]

Keeping all of the other parameters fixed.

X!Tandem seems to run fine and so does xinteract. However, when I look
at the number of matched ions for any given spectrum, this field
always reports zero.

Is anyone else seeing this?
Is this a bug or have I forgotten some critical flag for analyzing ETD
data?

I'm running TPP v.4.3.1 on RHEL 5.2 (64-bit). The command I execute at
the terminal is: xinteract -Opd -drev output.XML

Any and all advice is welcomed.
Thanks