ERROR : Constraint function must return real value

I'm using genetic algorithm as my optimization solver but I keep getting this error and I don't know why.

??? Error using ==> constrValidate at 30 Constraint function must return real value.

any suggestion.

Answer 1

[Walter Roberson]

Your constraint function is returning a complex value (or possibly a NaN). Remember that the constraint function may be executed with values that are out of range, with it being the responsibility of the constraint function to say whether they are in range.

[Hamid]

thank you Walter, my constraint is something like this :

y(1)=x(1)*a;
y(2)=x(1)*b;
c(1)=y(1)-y(2);

I set a breakpoint and c(1) will be always NaN.

what do you recommend to get rid out this error?

thank you my friend.