The max-flow min-cut theorem states that the value of the maximum flow is equal to the minimum cut capacity.
It is possible that the max-flow and min-cut is equal to $\infty$.
However, reading Introduction to Linear Optimization by Bertsimas and Tsitsiklis, I get the impression that the max-flow and min-cut problems are dual to one another.
From duality theory, I know that, if the primal has finite optimal value then the dual has finite optimal value. Also, if the primal has unbounded optimal value, then the dual is infeasible and vice versa.
In the case of the max-flow and min-cut problems, how can they both have unbounded optimal value ??