Singular matrix error

[Tabacof]

Greetings

I'm trying to simulate two solar cells in series (each solar cell can
be modelled as a current source in series with a resistance and in
parallel with a diode) that receive different irradiation (meaning
their current sources will have different values). My circuit is as
follows:

n1 = cir.add_node('net1')
#First solar cell
cir['is1'] = IS(gnd, 1, i = 3)
cir['diode1'] = Diode(1, gnd)
cir['r1'] = R(1, 2, r=0.012)
#Second solar cell
cir['is2'] = IS(2, 3, i = 3.8)
cir['diode2'] = Diode(3, 2)
cir['r2'] = R(3, n1, r=0.012)
#Load
cir['volt2'] = VS(n1, gnd, v = 1)

dc = DC(cir)
res = dc.solve()

When I run the code above, I get this:

WARNING:root:Problems encoutered: Singular matrix
WARNING:root:Problems encoutered: No convergence. xerror = [ 0.0000e
+00 2.5843e-02 2.5843e-02
4.2633e-17 0.0000e+00]
WARNING:root:Problems encoutered: Singular matrix
Traceback (most recent call last):
File "rc.py", line 25, in <module>
res = dc.solve()
File "c:\Python25\project\pycircuit\circuit\dcanalysis.py", line 76,
in solve
raise last_e
pycircuit.circuit.analysis.SingularMatrix: Singular matrix

Just to make it clear, if I simulate using the same current it will
work just fine, so I don't think the circuit itself is wrong. I
understand it is unusual to put two current sources with different
currents in series, but that's how it's done in solar cell modelling.
PSpice doesn't have a problem with that, for example.

Any help would be much appreciated!
Pedro Tabacof.