Error of savemesh in 3D


I install windows version FreeFem 4.11 in win 10. But there is something wrong when I save 3D mesh.
So I test the following simple code:

load "msh3"
load "medit"
mesh3 Th3=cube(10,15,5);
savemesh(Th3, "surf.mesh");

The program is terminated when it runs to savemesh, then I get a empty surf.mesh file with 0KB, I don’t know what the problem is?

Thank you in advance.