"Has improper value for" is a predefined property that tracks input errors for irregular value annotations and is provided by Semantic MediaWiki. In most cases it is caused by a type mismatch or a value restriction. This property is pre-deployed (also known as special property) and comes with additional administrative privileges but can be used just like any other user-defined property.
This is the page of special property Has improper value for. It lists all pages using this special property. See the help page Special property Has improper value for for further information.