R help - May 2013 - eRm package, RSM: Error in solve.default(parets$hessian) :

I want to make an index using IRT, I use eRm package.
I have a data frame (24 items, 4 categories, 427 cases) that works if I use
PCM (partial credit model), but, when I run RSM (rating scale model) this
appear:

Error in solve.default(parest$hessian) :
  Lapack routine dgesv: system is exactly singular: U[14,14] = 0

By mistake I recoded the items in three categories, and works the RSM, but
when I realized I recoded to four again and nothing... the same error.

Anyone?



THANKS!!!

	[[alternative HTML version deleted]]