LocalParameterization exception

[lsorgi]

I can't manage to make the LocalParameterization work.

Here is my LocalParameterization class definition

class SO3Parameterization(PyCeres.LocalParameterization):

def __init__(self):
    super().__init__()

def Plus(self, x, delta, x_plus_delta):
    #...compute step
    return True

def GlobalSize(self):
    return 4

def LocalSize(self):
    return 3

def ComputeJacobian(x, jacobian):
    # ...compute Jacobian
    return True

when I try to add the parameter block to the problem an exception is raised.

  problem.AddParameterBlock(r_params, 4, SO3Parameterization())

Am I using the LocalParameterization incorrectly?