the iphone could not be restored. An unknown error occured (1015)

i tried to update my iphone 3g with 3.1.3 to ios 4.2 version but i think it failed because it everytime I connect my iphone to the computer using itunes, it says "itunes has detected an iphone in recovery mode. You must restore before it can be used eith itunes". And when I did this instruction, after verifying firmware of my iphone, it says" the iphone could not be restored. An unknown error ocurred (1015). How can I cope with this?