Most problems are detected when Inform translates the source text, but this is one of the exceptions, a "run-time problem" which emerges only during play.

When a relation is defined, it has to say which objects are allowed to be related: for instance, Propinquity relates various rooms to various rooms indicates that both of the objects involved in the relation must be rooms. It would make no sense to declare that a container and a person now satisfy the relation, and the attempt would be rejected by a run-time problem like this one.

More is true: it is not even allowed to test the relation for objects which do not meet its qualifications. So if we define the adjective homely to refer to any room which satisfies the propinquity relation with a special room called Home Base, say, then we are allowed to say the list of homely rooms but not to say the list of homely things.