P11 - Never-set property

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.

It sometimes happens that - although something could in principle have a given property - in fact it doesn't. That makes it impossible to change this property.

This is a problem which really only occurs when Inform 6 code is mixed in with modern Inform style, and that might mean that - unless you're using Inform 6 yourself - you are actually having trouble with an extension which happens to use Inform 6 tricks in order to work.