NG Spice current reference

Ok so i've written this code to plot the I-V characteristic of a NMOS transistor :


.include "0.25_Cmos_Models"
.option scale=2u

M1001 2 1 0 0 CMOSN w=3u l=2u
+ ad=19p pd=18u as=19p ps=18u

Vgs 1 0 dc 0v
Vdd 3 0 dc 0v

*zero voltage source to measure current
Vz 2 3 dc 0v
.dc Vdd 0v 2.5v 0.25v *Vvgs 0v 2.5v 0.25v

.control
run
plot Vz#branch v(3)
.endc

.end

Then i get this error :
Circuit:

Scale set
Doing analysis at TEMP = 27.000000 and TNOM = 27.000000

Fatal error: m1001: effective channel length less than zero
doAnalyses: parameter value out of range or the wrong type

run simulation(s) aborted
Error: no such vector vz#branch

How is it possible to calculate the current? I'm using ngspice-26