The problem probably comes from the use of pm.Constant. There is no PyMC sampler that is specialized for it, and the generic samplers are likely to suggest other values which will have zero probability.
Is there any reason why you are using it? It’s mostly an internal distribution and users shouldn’t really need it outside of specialized cases like in Mixtures.
In your example you can simply use python numerical values as constants. If you want something that shows in graphviz you can use pm.Data instead.