[Ipopt] How to define a non-zero constraint?

[Lewis I]

Hello, 

I am try to define a non-zero constraint. i.e. a!=0
I am trying to use:     a*a-0.000001>=0.
where 0.000001 is the tolerance from zero.
But I found the outcome was not so good.
Do anyone has any ideas on modeling a non-zero constraint. 
Thank you so much

Regards,  
Lewis