The schema at http://www.xbrl.org/2006/xbrldi-2006.xsd does not make use of the id attribute on its schema elements. Gepsio leaves the value of the Id property of the JeffFerguson.Gepsio.Xsd.Element object as null when objects are hydrated from schema element definitions without ID attributes. Since the implementation of GetHashCode() for the JeffFerguson.Gepsio.Xsd.Element object relies on a non-null Id property value, code that uses the GetHashCode() method will crash with an object reference exception.
Gepsio should ensure that the Id property on JeffFerguson.Gepsio.Xsd.Element objects is always non-null and should also use a different property, perhaps Name, in its GetHashCode() implementation so that hash uniqueness can be assured.