How to solve the problem"Attempt to evaluate non-integral power of negative number. Function: ^"

Hello,

I am solving a Drift diffusion model for plasma using general form PDE. However, I am facing the following error,

Failed to find consistent initial values.
Attempt to evaluate non-integral power of negative number.
Function: ^
Failed to evaluate variable.
Variable: mod1.uth, Defined as: (((mod1.Te*(k B const*8))/(pi*mod1.me))^0.5)
Failed to evaluate expression.
Expression: d((0.5*mod1.uth*mod1.u*test(mod1.u))*(dvol),{test0})
Last time step is not converged.


Here, Te= (2/3)*(n/ne)
n, ne is the dependent variables for the problem and their initial ratio is 3. So it is positive. And also k_B_const is Boltzman constant is a constant, me is the mass of electron also a positive quantity.

Can you tell me why it happens?

Thanks is advance.

Rajib