I took the example from the website and changed it slightly. Now I get an error ("Power with free indices not allowed"). What am I doing wrong?

```
{u,r,\phi,\theta}::Coordinate;
{\mu,\nu,\rho,\sigma,\lambda,\kappa,\chi,\gamma}::Indices(values={u,r,\phi,\theta}, position=fixed);
\partial{#}::PartialDerivative;
g_{\mu\nu}::Metric.
g^{\mu\nu}::InverseMetric.
ss:= { g_{u u} = -(1-r^2/l^2),
g_{u r} = -1,
g_{r u} = -1,
g_{\theta\theta} = r**2,
g_{\phi\phi}=r**2 \sin(\theta)**2
}.
complete(ss, $g^{\mu\nu}$);
ch:= \Gamma^{\mu}_{\nu\rho} = 1/2 g^{\mu\sigma} (
\partial_{\rho}{g_{\nu\sigma}}
+\partial_{\nu}{g_{\rho\sigma}}
-\partial_{\sigma}{g_{\nu\rho}} ):
evaluate(ch, ss, rhsonly=True);
```