Error in ocean script!!!

[Samiran]

Dear all,

I am trying to run a batch simulation through ocean script where
dependint upon input signal frequency transient analysis simulation
time will be fixed. For this my approach is:

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////
....
stop_time = 1/freq ; freq is a design variable which changes in
each iteration.
....
analysis('tran ?start 0 ?stop stop_time ?errpreset "conservative" ?
write "spectre.ic" ?writefinal "spectre.fc" ?annotate "status" ?
finalTimeOp t ?maxiters "5" ?threshold "0.0" ?detail "node" ?sort
"name" )
....

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////

But when I run the script I am getting an error:

Error found by spectre during transient analysis `tran'.
Stop time must follow start.
Analysis `tran' terminated prematurely due to error.


But when I hardcode the stop_time inside the script like I have shown
below, it is working fine.

stop_time = 0.2m


Please help me!!!

Regards
Samiran