The section says The result of the function must be suitable for the invocation context. which is the problem obviously. The invocation context "select item" can only be suitable if we know the type of the expression. The only way to tell Hibernate that type, is to register the function in the Dialect. I suppose the JPA spec deliberately left that open to not impose the requirement to know all functions on the JPA provider. Anyway, you can easily fix the problem yourself and I guess other JPA providers behave similarly. You might just have had bad luck by using a function that isn't included in the HSQLDB dialect yet.