userName está undefined. Será que estou fazendo algo errado com o ActivatedRoute?

Ao executar: GET http://localhost:4200/user/flavio Ocorre erro: GET http://localhost:3000/undefined/photos 404 (Not Found)

app.routing.module.ts

import { NgModule } from '@angular/core';
import { Routes, RouterModule } from '@angular/router';

import { PhotoListComponent } from './photos/photo-list/photo-list.component';
import { PhotoFormComponent } from './photos/photo-form/photo-form.component';
import { NotFoundComponent } from './errors/not-found/not-found.component';

const routes: Routes = [
  { path: 'user/:userName:', component: PhotoListComponent },
  { path: 'p/add', component: PhotoFormComponent },
  { path: '**', component: NotFoundComponent }
];

@NgModule({
  imports: [RouterModule.forRoot(routes)],
  exports: [RouterModule]
})
export class AppRoutingModule { }

photo-list.component.ts

import { Component, OnInit } from '@angular/core';
import { ActivatedRoute } from '@angular/router';

import { PhotoService } from '../photo/photo.service';

@Component({
  selector: 'app-photo-list',
  templateUrl: './photo-list.component.html',
  styleUrls: ['./photo-list.component.css']
})
export class PhotoListComponent implements OnInit {

  photos = [];

  constructor(
    private photoService: PhotoService,
    private activatedRoute: ActivatedRoute
  ) { }

  ngOnInit(): void {

    const userName = this.activatedRoute.snapshot.params.userName;

    this.photoService
      .listFromUser(userName)
      .subscribe(photos => this.photos = photos);
  }
}

photo.service.ts

import { HttpClient } from '@angular/common/http';
import { Injectable } from '@angular/core';
import { Photo } from './photo';

const API = 'http://localhost:3000';

@Injectable({ providedIn: 'root' })
export class PhotoService {

  constructor(private http: HttpClient) { }

  listFromUser(userName: string) {

    console.log('userName', userName);

    return this.http
      // .get<Photo[]>(API + '/' + 'flavio' + '/photos');
      .get<Photo[]>(API + '/' + userName + '/photos');
  }
}