2.1.11 [ECMA-262-1999] Section 8.6.2, Internal Properties and Methods


The value of the [[Prototype]] property must be either an object or null, and every [[Prototype]] chain must have finite length (that is, starting from any object, recursively accessing the [[Prototype]] property must eventually lead to a null value). Whether or not a native object can have a host object as its [[Prototype]] depends on the implementation.

JScript 5.x does not permit a native object to have a host object as its [[Prototype]].