Absolute Value in optimization expression

How can I add an absolute value to an optimization expression I am trying to optimize? MATLAB shows the following error :

Undefined function 'abs' for input arguments of type 'optim.problemdef.OptimizationExpression'.

problem is :

minimize abs ( ax + by + cz - d )

subject to , x,y,z are integers in some range

Answer 1

[Torsten]

min: eps
+ (a*x + b*y + c*z - d) - eps <= 0
- (a*x + b*y + c*z - d) - eps <= 0

subject to x,y,z are integers in some range with a new variable "eps".

Answer 2

[Sean de Wolski]

Square it instead so as to not introduce a discontinuity

(expr).^2