# Mesh not appearing on 3d plot

I'm trying to get a mesh to appear on a surface in Sage 4.8 using `parametric_plot3d`

. Here is the code:

```
var('u,v')
f=[u-u^3/3+u*v^2,-v+v^3/3-v*u^2,u^2-v^2]
parametric_plot3d(f,(u,-7,7),(v,-7,7),opacity=0.7,mesh=True)
```

What am I doing wrong? I also can't get any of the examples from the tutorial page to show a mesh.