# Tangent vector field

Hello over there.

I'm trying to calculate the norm, or the norm squared, of a vector field tangent to a curve over a manifold. The examples on **Curves in Manifold** and **Vector Fields** from the documentation work fine, but when I try a tangent to a curve I get the error `ValueError: the two subsets do not belong to the same manifold`

.

Here is my minimal example:

```
N = Manifold(2, 'N', latex_name=r'\mathcal{N}',structure='Lorentzian')
var('u v')
chart_N.<u,v> = N.chart()
R.<t> = RealLine()
beta = N.curve({chart_N: [t, sech(t)]}, (t,0, oo), latex_name=r'\beta')
vbeta = beta.tangent_vector_field()
g=N.metric(name='g', latex_name=r'g_{\mathcal{N}}')
g[0,0]=-1
g[1,1]=cosh(u)**2
```

Everything is fine until here. I got the error when I tried

```
g(vbeta,vbeta)
```

or

```
vbeta.norm(metric=g)
```

What I'm missing?

Thank you in advance.