[arm-gnu] Assembler error for cortex-m3

[Chris Elliott <chris@xxxxxxxxxxxxxxxxxxxx>]

I am compiling some assembler code for cortex-m3 and get an error.
The same error occurs with either gcc or as.

arm-2011.03/bin/arm-none-eabi-gcc -mcpu=cortex-m3 -march=armv7-m -c -o
test.o startup_LPC17xx.s
startup_LPC17xx.s:138: Error: Thumb does not support this addressing
mode -- `strb R2,[R0],#1'

This addressing mode is listed as supported in my reference documents
for the LPC1768 processor.
Any idea why the compiler doesn't think it is supported?

Thanks,
-Chris