itensor, ic_convert for continuity equation

Dear all,
I am trying to get the Navier Stokes equation into MAXIMA using the
tensorial packages.
I am starting by writing the continuity eq

d rho / d t ?+ (rho v^ i )_{ ,j } = 0

or in latex
\frac{\partial \rho}{\partial t} + (\rho v^i)_{,j} ? (
http://tinyurl.com/ydxd6pc )

The idea is to start with "itensor" and then convert it to "ctensor"
defining the metrics in order to arrive to a final equation in a
defined coordinate system.

derivabbrev:true$
load(itensor)$
load(ctensor)$
ct_coordsys(cartesian3d)$
cmetric()$
christof(mcs)$
depends([rho,p,q,T],[ct_coords, t])$
imetric: g$
eq: ishow( s([a],[]) =  covdiff((rho([],[]) * q([a],[])),b))$
ic_convert(eq);
s: [0,0,0]$
%th(2), ev;

I get::
 "Improper indices in - rho([], []) q([%3], []) ichr2([a, b], [%3])
                        + rho([], []) q([a], [], b) + rho([], [], b) q([a], [])
 -- an error.  To debug this try debugmode(true);"

I use the same indices both in LHS and RHS and "s" is a dummy var to
give value zero to the LHS. it is basically the same thing done in
"Maxima Digest, Vol 41, Issue 37" message 5 by Viktor T. Toth.
Do you know where I am wrong?

Regards

Fabio