I have a Nexus S (2.3.3, GRI54/I9023, bootloader unlocked, clockworkmod recovery installed, rooted) and I'm trying to flash it with a stock system.img that I built myself (no modifications - yet).
To make the stock Android build, I followed the instructions for making a build found at http://source.android.com/source/download.html. The build was successful and the system does work when run on the emulator.
To flash the ROM, I connect the phone, boot with Volume up + Power, connect USB and type
fastboot flash system system.img
The response that is shown on the phone is
Invalid magic code 0x00000003
What does this mean? What's wrong?