Flashing recovery fails "due to S-ON"

I get the following error when trying to flash a recovery image onto the phone. Fastboot works well, but here is the error I get afterwards:

platform-tools malika$ /Users/malika/android_sdk/platform-tools/fastboot flash recovery twrp.img
Sending ‘recovery’ (23552 KB) OKAY [ 1.246s]
Writing ‘recovery’ (bootloader) Image verification failed…

FAILED (remote: 'Err! Stop flashing due to S-ON…
fastboot: error: Command failed

Any help would very welcome!

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A brief search suggests this may provide the answer?

s-on and want to flash roms? READ THIS - HTC Rezound | Android Forums