expectation value of a noisy system

Question

I'm trying to find the expectation value of an operator using a noisy circuit and this is the code I found:

obs = Observable(PauliString("Z"))

prob_1 = 0.005 

prob_2 = 0.01  
 
error_1 = noise.depolarizing_error(prob_1, 1)

noise_model = noise.NoiseModel()

noise_model.add_all_qubit_quantum_error(error_1, ['u1', 'u2', 'u3'])

def sim_noise(qc, obs : npt.NDArray[np.complex64])-> float:
    return execute_with_shots_and_noise(qc, obs, noise_model , 4096)

def sim(qc, obs : npt.NDArray[np.complex64])-> float:
    return execute_with_shots(qc, obs, 4096)

ideal_measurement = obs.expectation(get_circuit(), sim_noise).real.

I keep getting this error:

ValueError: Executor and observable are not compatible. Executors
                returning expectation values as float must be used with
                observable=None.

I couldn't figure out how to fix it.