Negative Concentrations

Hello,

How to avoid negative concentrations? Can anyone please help me?

dCA/dt = -CA1*ko*exp(-E/R/T)

I used the following constraint on concentration.

CA1=if(CA<0,0[mol/m^3],CA)

But it returns the following error
Recoverable error in residual, but solver could not recover.
Time : 0.03270013548094004
Last time step is not converged.

Thanks
Krish