Setting vhdl tb generic using irun command

Hi all,

I've got a problem. In my tb I've got a generic:
        G_WAVEFORM_TYPE : string := "xxx" 

and i'd like to change "xxx" to "yyy" using irun "start.f" file, but i can't

-mess -v93 -assert ../src/dut.vhd ../src/tb.vhd -generic "tb.G_WAVEFORM_TYPE => "yyy""
or in other file:
-top tb -access rwc -defparam tb.G_WAVEFORM_TYPE="yyy" -nowarn CUDEFB
doesn't work. Could you  tell me how to set a tb (vhdl) generic from *.f file using irun -f and what is the correct command to do it?

Thanks,
John.