TypeError: Cannot read property 'map' of undefined

ı am using angular full calendar and ı get this error

service.ts

 getEvents()
  {
    return this.http.get('http:///api/llessons') .map( response => {
      return response.json().data.map((data) => {
            return {...data,start: data.start_time};
          });
   }); 
  }

home.ts

 ngOnInit(){

   

    this.events = this.service.getEvents().subscribe(events => {this.events = events;});
    console.log(this.events);

    
}

ı faced this error TypeError: Cannot read property ‘map’ of undefined so please help me.