Editing Formula for a family

I have created a shared parameter X, and I'm just applying a simple logic such as

IF(X<0, X=0, X)

In order to prevent negative value input.

However, it shows up a warning "There is a circular chain of references among the formulas"

If I change it to 

IF(X<0,X=0, X)

There is another warning "Inconsistent units"

What is wrong actually and what should I do ?

Cause in the future I want to assign the logic like

IF(X<0, AND(Y=0-X, X=0), X=X)

THank you