CompleteMeasFitter leads to error in QuantumInstance

[lasys]

Information

• Qiskit Terra version: 0.18.0
• Qiskit version: 0.28.0
• Python version: 3.6.9
• Operating system: Linux

What is the current behavior?

quantum_instance = QuantumInstance( 
                                        backend=Aer.get_backend('qasm_simulator'), 
                                        shots=8000,
                                        noise_model=noise_model,
                                        coupling_map=coupling_map,
                                        basis_gates=basis_gates,
                                        measurement_error_mitigation_cls=CompleteMeasFitter,
                                        measurement_error_mitigation_shots=8000
                                      )         
 

leads to:

~/.local/lib/python3.6/site-packages/qiskit/utils/quantum_instance.py in execute(self, circuits, had_transpiled)
    608                     if curr_qubit_index == qubit_index:
    609                         tmp_fitter = meas_error_mitigation_fitter
--> 610                     elif isinstance(meas_error_mitigation_fitter, CompleteMeasFitter):
    611                         tmp_fitter = meas_error_mitigation_fitter.subset_fitter(curr_qubit_index)
    612                     else:

UnboundLocalError: local variable 'CompleteMeasFitter' referenced before assignment

Steps to reproduce the problem

    quantum_instance = QuantumInstance( 
                                        backend=Aer.get_backend('qasm_simulator'), 
                                        shots=8000,
                                        noise_model=noise_model,
                                        coupling_map=coupling_map,
                                        basis_gates=basis_gates,
                                        measurement_error_mitigation_cls=CompleteMeasFitter,
                                        measurement_error_mitigation_shots=8000
                                      )                 
    
    qaoa = QAOA(optimizer=optimizer, 
                quantum_instance=quantum_instance, 
                reps=reps,
                initial_point=init_parameters) 

    algorithm = MinimumEigenOptimizer(qaoa)
    result = algorithm.solve(problem)

What is the expected behavior?

No error message.
Same code is working fine with terra version 0.17.4 on Qiskit version 0.27.0.

Suggested solutions

None, but error only occurs when coupling_map and/or basis_gates are set.