Nonlinear capacitance definition

Hi,

 I have a question what is wrong in below nonlinear capacitance definition for C_TBU, which defines capacitance as a function of the capacitance voltage:
C_TBU Node1 Node2 8n*(3.3^2)/((3.3^2)+(v(C_TBU)^2))

Thanks for any help,

Andrew