Can't find eigenvalues



I'm trying to find the eigenvalues of a simple matrix

b:matrix([0,-1/4,0,0],[1/4,0,0,0],[0,0,0,-1/4],[0,0,1/4,0]);

by

eigenvalues(b);

But Maxima give me

part fell off end.
#0: eigenvalues(mat=matrix([0,-1/4,0,0],[1/4,0,0,0],[0,0,0,-1/4],[0,0,1/4,0]))(eigen.mac line 94)
-- an error.  To debug this try debugmode(true);

What wrong with this? Can anyone help?

Noparit Jinuntuya
Department of Physics, Kasetsart University
Thailand.