help on the classic differential amplifier LM 339 (opamp)

[Morten]

Hi, I am using the LM339 in a circuit like shown below. The LM339 is a
single supply opamp.

_____R________
| |
| ------- |
V1 --R-------|- - |
| -------------- Vout
V2 --R-------|+ -
| -------
|
R
|
gnd


Opamp supply: +12 V and ground.
V1=0
V2=6V
R=100 kohm

With these values I get Vout = 0V!!! . I expected vout to be equal V2
because of unity gain. What am I doing wrong. Is it the LM339 that cant be
used in the circuit?