unable to find cholesky factorization



with wxmaxima 0.7.1 ubuntu hardy
D:matrix([a,b],[b,c]);
cholesky(D);
Unable to find the Cholesky factorization -- an error.  To debug this 
try debugmode(true);

even with

assume(determinant(D)>0);
cholesky(D);
Unable to find Cholesky etc...

but

cholesky(transpose(D).D);
Is  b^2+a^2  positive or zero?positive;
result...
which means cholesky symbolic works..