on()函数创建Reducer时出错

import { createReducer, on, Action } from '@ngrx/store';
import * as ProvaActions from './prova.actions';
import { ProvaState } from './prova.model';


export const initialState: ProvaState = {
    user: {},
    error: false
}

const provaReducer = createReducer(
    initialState,
    on(ProvaActions.editProvaSuccess, (state, action ) => ({user: action.user , error: false })),
    on(ProvaActions.editProvaFailed, (state, action ) => ({ user: {}, error: true }))
);
  
export function reducer(state: ProvaState | undefined, action: Action) {
    return provaReducer(state, action);
  }

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