Afaik, built-in parameter enumeration values are negative constants and are sometimes (mis-) used as pseudo element ids.
Have you checked whether the negative integer value corresponds with a built-in parameter enum?
Possibly related discussions:
http://thebuildingcoder.typepad.com/blog/2010/04/elementid-parameter-value.html
"element ids are used to represent elements, parameters and categories. for params, the id may be negative, e.g. bip, or non-negative, custom or shared. when you call get element on that id, it returns the parameter definition."