Importing saved 3d plot

Hi everyone,

I generated a bunch of 3d plots to produce an animation and I saved them both as png and sobj files, because I wanted to keep the objects for later manipulations. However, when loading back an sobj file, I find them to be unusable. More precesily,

p=plot3d(lambda x1,y1: h(t0,x1,y1), (-5,5),(-5,5),plot_points=100); #t0 fixed and h(t,x,y) a procedure
p.save('bump003.sobj');
p.save('bump003.png');
a = load('bump003.sobj');
a.show();

returns the error

NotImplementedError: You must override the get_grid method.

while the png image files get correctly generated.

There is no mention of this kind of error in the Plot3D doc, except for parametric surfaces

http://www.sagemath.org/doc/reference/sage/plot/plot3d/parametric_surface.html

where it is mentioned that get_grid should indeed be overriden for subclasses of parametric_surface. Any idea why this error shows up only after importing the object ?

Many thanks, Benhuard