How to use REPLACE function

Hi! I can’t make the REPLACE function work, it returns nothing, example:

exten => 100,1,NoOp() same => n,Set(test=hello you) same => n,Verbose(1, string 1: ${test}) ; prints => string 1: hello you same => n,Set(test=${REPLACE(${test},o,u)}) same => n,Verbose(1, string 2: ${test}) ; prints => string 2: same => n,Hangup()

I have 10.7.1 version of Asterisk, running on Ubuntu 12.04.1 LTS

I found out thanks to the help of people in IRC that I need to give the variable name in the first argument and not the variable value as I did.
Works now:)