When you have an expression as a divisor in a CP Optimizer model, the divisor can be evaluated as zero.
This is part of the expected behavior and has been documented in CP Optimizer under the
operator/ concert function section. Click here to read more about this in CPLEX Optimization Studio 12.4.
Resolving the problem
If you need to guarantee the divisor to be non-zero, then you will need to formulate a constraint to enforce it explicitly.