Vector3D.Reject only work when direction vector is of unit length

[yunsguo]

    public static void Reject(ref Vector3D vector, ref Vector3D direction, out Vector3D result)
    {
        //  Optimized: float inv_denom = 1.0f / Vector3.Dot(normal, normal);
        double invDenom;
        Vector3D.Dot(ref direction, ref direction, out invDenom);
        invDenom = 1.0 / invDenom;
        // meaning invDenom = 1 / b·b = 1 / |b|*|b|*cos(0) = 1 / b^2

        //  Optimized: float d = Vector3.Dot(normal, p) * inv_denom;
        double d;
        Vector3D.Dot(ref direction, ref vector, out d);
        d = d * invDenom;
        // meaning d = a·b * invDenom = a·b / b^2

        //  Optimized: Vector3 n = normal * inv_denom;
        Vector3D n;
        n.X = direction.X * invDenom;
        n.Y = direction.Y * invDenom;
        n.Z = direction.Z * invDenom;

        //  Optimized: return p - d * n;
        result.X = vector.X - d * n.X;
        result.Y = vector.Y - d * n.Y;
        result.Z = vector.Z - d * n.Z;
        // meaning result = a - b * d / b^2 = a - b * a·b / b^4 
        // where a is the target vector and b is the direction
    }

What this does is that it returns a - b * (a·b) / b^4 where a is the target vector and b is the direction vector.
The rejection of vector a from vector b should be a - b * (a · b) / b^2, and the only way the function works is when b^2 =b^4