This doesn't work for me:

```
In [11]: p1.parameters = [0,1,2,3,4]
ERROR: InputParameterError [astropy.models.parameters]
---------------------------------------------------------------------------
InputParameterError                       Traceback (most recent call last)
<ipython-input-11-b31655922c88> in <module>()
----> 1 p1.parameters = [0,1,2,3,4]

/Users/tom/Library/Python/3.2/lib/python/site-packages/astropy/models/models.py in parameters(self, value)
    390         elif isinstance(value, (list, np.ndarray)):
    391             _val = parameters._tofloat(value)
--> 392             if self._parameters._is_same_length(_val):
    393                 self._parameters._changed = True
    394                 self._parameters[:] = _val

/Users/tom/Library/Python/3.2/lib/python/site-packages/astropy/models/parameters.py in _is_same_length(self, newpars)
    291 
    292         """
--> 293         parsize = _tofloat(newpars).size
    294         return parsize == self.__len__()
    295 

/Users/tom/Library/Python/3.2/lib/python/site-packages/astropy/models/parameters.py in _tofloat(value)
     35     else:
     36         raise InputParameterError(
---> 37             "Don't know how to convert parameter to float")
     38     return _value
     39 

InputParameterError: Don't know how to convert parameter to float
```