Dynamics AX: No metadata class defined for data contract object

When creating and using services you can get the following error:

No metadata class defined for data contract object

When you get this error it means that you are trying to use a type as a data contract that is not a valid data contract type.
Valid type are primitive types, tables and views, data contracts, containers, EDT’s, and collections (For a full list of supported types, check the Using Data Contracts in X++ [AX 2012] page on msdn.