Programming error

I am trying to classify certain objects in a room as expensive and am forbidding the player from taking that item.

Worth is a kind of value. The worths are good, bad, and ugly. Definition: a thing is expensive if it is good. Definition: a thing is cheap if it is bad. A gift is a kind of thing. A gift has a worth. Instead of taking a thing which is expensive: say "Your insurance would never cover something that expensive."

However, I now get the following error message in the game whenever the player attempts to pick up a container (not classified with the worth value)

[code]>take mirror

[** Programming error: object#57 (object number 57) has no property p57_worth to read **]
Your insurance would never cover something that expensive.[/code]

You can add to the definitions so that things not classified by worth are neither expensive nor cheap; that should fix the errors:

Definition: a thing is expensive if it provides the property worth and it is good. Definition: a thing is cheap if it provides the property worth and it is bad.

Or:

Definition: a thing is expensive if it is a gift and it is good.

Or even:

Definition: a gift is expensive if it is good.

(Anything not a gift will not qualify as expensive.)