Assembler Problem

I’m trying to compile a bit of assembly code for an ARM Cortex-M3 (which is Thumb-2 only), and the assembler is puking on the following two lines:

stmia r0,{r4-r11}

ldmia r0,{r4-r11}

Both lines generate an “Error: lo register required” message.

Any ideas as to why this isn’t assembling?