Error_msg

----> 1 metric_to_Christoffel_2nd(metric)

/usr/local/lib/python2.7/dist-packages/sympy-0.7.7.dev0-py2.7.egg/sympy/diffgeom/diffgeom.pyc in metric_to_Christoffel_2nd(expr)
   1535     s_fields = list(s_fields)
   1536     dums = coord_sys._dummies
-> 1537     matrix = matrix.subs(list(zip(s_fields, dums))).inv().subs(list(zip(dums, s_fields)))
   1538     # XXX end of workaround
   1539     christoffel = [[[Add(*[matrix[i, l]*ch_1st[l][j][k] for l in indices])

/usr/local/lib/python2.7/dist-packages/sympy-0.7.7.dev0-py2.7.egg/sympy/matrices/matrices.pyc in inv(self, method, **kwargs)
    311         if method is not None:
    312             kwargs['method'] = method
--> 313         return self._eval_inverse(**kwargs)
    314
    315     def inv_mod(self, m):

/usr/local/lib/python2.7/dist-packages/sympy-0.7.7.dev0-py2.7.egg/sympy/matrices/dense.pyc in _eval_inverse(self, **kwargs)
    305         M = self.as_mutable()
    306         if method == "GE":
--> 307             rv = M.inverse_GE(iszerofunc=iszerofunc)
    308         elif method == "LU":
    309             rv = M.inverse_LU(iszerofunc=iszerofunc)

/usr/local/lib/python2.7/dist-packages/sympy-0.7.7.dev0-py2.7.egg/sympy/matrices/matrices.pyc in inverse_GE(self, iszerofunc)
   2641         red = big.rref(iszerofunc=iszerofunc, simplify=True)[0]
   2642         if any(iszerofunc(red[j, j]) for j in range(red.rows)):
-> 2643             raise ValueError("Matrix det == 0; not invertible.")
   2644
   2645         return self._new(red[:, big.rows:])