Image rotation

I am attempting to use the warp affine function in VisionWorks, except, using negative numbers to rotate generates a black image. Here is the code:

vx_float32 a = -1.0f, b = 0.0f, c = 0.0f, d = 0.0f, e = -1.0f , f = 0.0f;
//! [warp affine]
// x0 = a x + b y + c;
// y0 = d x + e y + f;
vx_float32 mat[3][2] = {
{a, d}, // ‘x’ coefficients
{b, e}, // ‘y’ coefficients
{c, f}, // ‘offsets’
};
vx_matrix matrix_rot = vxCreateMatrix(context, VX_TYPE_FLOAT32, 2, 3);
vxWriteMatrix(matrix_rot, mat);
vx_status status = vxuWarpAffine(context, src, matrix_rot, VX_INTERPOLATION_TYPE_NEAREST_NEIGHBOR, dst);

Is there an issue with the implementation?