How to run spectremdl about stb analysis?

I create a stb.scs file ,and run "spectre stb.scs",
the result is correct, the loop gain, gain margin, phase margin are all right.

However, during the spectremdl simulation, errors come up:
FATAL(CMI-2010):Assertion failed in file 'dfistrm.c' at line 266.
FATAL (SPECTRE-21):Assertion failed.
Aborted ${APPBINPATH}/${PROGRAM}"$@"0<&3


The stb.scs:
.......................
......................
itest (vp fb) iprobe
stbAnalysis stb start=1 stop=1e9 dec=20 probe=itest

Alter_tt altergroup{
include "./model.scs" section=tt}

Alter_ff altergroup{
include "./model.scs" section=ff}
................................
...............................


The stb.mdl:
alias measurement stbrun{
run stbAnalysis
}

foreach vdd from {3.3}{
run Alter_tt
run stbrun as stbrun_tt

run Alter_ff
run stbrun as stbrun_ff
}

Another question is how to manage the mdl file in order
to measure the loopgain, phase margin, gain margin directly?

Thanks in advance!