Nibble mod

Hello all
Can you help me. i am setting up mod_nibble. after i enabled the module, the extension table has a new column nibble_account. i added by my self another column for nibble_rate. then i added the below 2 lines in my outbound route :
<action application="set" data="nibble_rate=${nibble_rate}"/>
<action application="set" data="nibble_account=${nibble_account}"/>

but this not working. when i changed the nibble_rate to be a fixed number such as "nibble_rate=5.0" , it works fine and deduct amount.
how i can set nibble_rate per extension?
Thank you.