FFT in SpectreMDL

I'm trying to use the FFT to analyze a transient waveform and
extract the signal to noise ratio. I'm using MMSIM6.2, with the mdl

alias measurement tran_run {
run analysisTRAN1
// 1) Extract 'snr_fft'
export real Zsnr_fft0 = snr(fft(sig=V(v_out), from=0ns, to=160ns,
numpoints=1024, window='rectangular), \ sig_from=11e6, sig_to=13e6,
noise_from=1, noise_to=2e9)
run tran_run as tran1

The simulation executes, but the function fft is not found. From the
output of "spectremdl -batch filename.mdl -design filename.scs":

Error found by spectre during transient analysis `tran1', during task
ERROR: line `8': Unable to bind function `fft'.
ERROR: Argument 'sig' cannot be cast scalar to waveform.
ERROR: line `8': Operand 'fft' of function 'snr' contains an
ERROR: line `8': The function 'snr' contains no arguments

Is it possible to use the fft function when MDL is used in batch
mode? Is there some other problem I'm not aware of? Thanks!!!