Boundary condition jacobian with bvp4c

Hi

I'm solving a system of 2nd order ODE's

y'' = f (t, y, y')

with the boundary conditions

y (0) = a, y (1) = b

For this I am using bvp4c with success. I would like to supply analytic Jacobians, so I started with these for the boundary conditions. I encode the boundary conditions as

 bc = @(p1, p2) boundary (p1, p2, a, b);

with

 function bc = boundary (p1, p2, a, b)
   D = numel (p1_goal);
   d1 = p1 (1:D) - a (:);
   d2 = p2 (1:D) - b (:);
   bc = [d1; d2];
 end % function

As I am solving a 2nd order system my state is (y; y'). I would assume that

partial boundary / partial y  = identy matrix
partial boundary / partial y' = zero matrix

but when I give this to bvp4c, I get an error saying that the Jacobian is singular (which is true).

Can anybody tell me how to correctly provide the Jacobian of the boundary conditions?

Thanks Soren