MIPS assembly error

[Jeff Baker]

I posted this question a while ago and didn't receive a response from 
the list, so I'll try again.

Assemble the following code for MIPS (binutils 2.15.91):
===================================
.data
actives: .word 0
ker_stack: .word 0

.equ distance,(ker_stack-actives)

testloc: .word distance

.text
li $4,distance
===================================

And you get:

main.s: Assembler messages:
main.s:10: Error: absolute expression required `li'

Is this a bug in gas?