问Reducer在ngrx中抛出“error object null is not iterable”

EN

import { createAction, props } from '@ngrx/store';
import { Doctors } from 'src/app/admin/models/doctorProfileAdmin'

export const loadDoctors = createAction('[Doctors] Doctors List Request', props<{ hospitalId: string }>())

export const loadDoctorsSuccess = createAction('[Doctors] Doctors List Load Success', props<{ doctors: Doctors[] }>())

export const loadDoctorsFail = createAction('[Doctors] Doctors List Load Fail', props<{ errorMessage: string }>())

export const addExistingDoctorToList = createAction('[Doctors] Doctors List Load Success', props<{ doctor: Doctors }>())

import { createReducer, on, Action } from '@ngrx/store';
import { DoctorListState } from '../state/doctorList.state';
import { loadDoctors, loadDoctorsSuccess, loadDoctorsFail,addExistingDoctorToList } from '../actions/doctorList.actions';


const initialState: DoctorListState = {
    doctors:null,
    error: null,
    isLoading: false
};

const doctorListReducer = createReducer(initialState,
    on(loadDoctors, (state) => {
        return { ...state, isLoading: true };
    }),
    on(loadDoctorsSuccess, (state, { doctors }) => {
        return { ...state, doctors: doctors, isLoading: false };
    }),
    on(loadDoctorsFail, ((state, { errorMessage }) => {
        return { ...state, errorMes: errorMessage, isLoading: false };
    })),
    on(addExistingDoctorToList, (state, { doctor }) => {
        return { ...state, doctors:[...state.doctors,doctor], isLoading: false };
    }),

);

export function reducer(state: DoctorListState | undefined, action: Action) {
    return doctorListReducer(state, action);
}

 this.store.pipe(select(selectDoctorsList)).pipe(skip(1)).subscribe(payload => {
        this.doctorList = payload;
 })

addNewDoctor(){
   this.store.dispatch(addExistingDoctorToList({doctor:selectedDoctor}))
}