P10 - Non-existent 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.

Not everything has every property. For instance, a room cannot have the property carrying capacity: it would not mean anything. Generally speaking, the properties available to something are specified by its kind. If we say that A room has a number called maximum population. then the property maximum population is available to any room. This applies to either/or properties as well: you might say that a room is not transparent, but Inform takes the firm line that we cannot even ask, because it is meaningless for a room to be either transparent or opaque.

So, anyway, here it seems that you have tried to access a property for something not allowed to have it.