Service Engine Soon Code P0440
 

I got the Service Engine Soon light on my BMW X5 2001. I was on my way to Auto Zone, and like the car knew the light went off. Anyways, I still had the chip read, and it said:
P0440
The PCM has determined that a malfunction exists in the evaporative emission control system.

I searched through the internet to decode this and am with no result.
I had the code cleared from Auto Zone.
Any body know about this issue and how I could rectify this problem?
Please suggest.