ModelSim VCD Error

Hey!

I have a problem by establishing a .vcd:

What I'm doing is first:

vsim inverter_tb
add wave *
vcd file myvcdfile.vcd
run 100ns

and then quit the simulation, BUT my .vcd file just contains the header, something like this:

$date

Thu Aug 04 12:49:07 2011

$end

$version

ModelSim Version 6.5e

$end

$timescale

1ns

$end

Thanks for suggestions