You are not showing the `pars` keyword in the following example.

I tried it and it doesn't work here:
```

In [111]: ch2 = models.ICheb2DModel(xdeg=2, ydeg=3, pars=coeff)
ERROR: AssertionError [astropy.models.models]
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-111-792e3098de53> in <module>()
----> 1 ch2 = models.ICheb2DModel(xdeg=2, ydeg=3, pars=coeff)

/Users/deil/code/astropy/astropy/models/models.pyc in __init__(self, xdeg, ydeg, xdomain, xwindow, ydomain, ywindow, paramdim, **pars)
   1074                                            xdomain=xdomain, ydomain=ydomain,
   1075                                            xwindow=xwindow, ywindow=ywindow,
-> 1076                                            paramdim=paramdim, **pars)
   1077 
   1078 

/Users/deil/code/astropy/astropy/models/models.pyc in __init__(self, xdeg, ydeg, xdomain, xwindow, ydomain, ywindow, paramdim, **pars)
    577                 print("Creating a model with {0} parameter sets\n".format(lenpars))
    578                 paramdim = lenpars
--> 579             self._validate_pars(**pars)
    580             self.set_coeff(pardim=paramdim, **pars)
    581         super(IModel, self).__init__(self.parnames, ndim=2, outdim=1,

/Users/deil/code/astropy/astropy/models/models.pyc in _validate_pars(self, **pars)
    617     def _validate_pars(self, **pars):
    618         numcoeff = self.get_numcoeff()
--> 619         assert(len(pars) == numcoeff)
    620 
    621     def _invlex(self):

AssertionError: 
```