Hi Glen,
    In the SMA Miriad beta version, there is a routine
(a debugging feature) to examine the header paremeters
and variables in handling SMA uv data. If this routine
detects problems in the ancillary variables which might cause
trouble or inaccuracies in calibrations and/or imaging, the relevant
uv-handling tasks will stop the process rather than produce
confusing results or collapse the programs.

    If one thinks that the problematic variables in the uv data
will make no bad effects, one can disable this header-check
function. In your case, for example, you can add an "options=nochk"
in invert to disable this function:

invert vis=grb_u.uv,grb_l.uv options=systemp,nochk cell=0.2 imsize=256
sup=0 map=grb.map beam=grb.beam

       Jun-Hui