Error while unit testing HttpClientModule (Angular 4.3+)

I'm exploring the new Angular HttpClientModule and running into an inexplicable error. The module is new enough that I can't yet find any useful information about how to unit test, and the official documentation doesn't have any examples.

The app contains a service with one method, which passes a URL to http.get. When I call this method in a browser context (aka ng serve), the http call executes normally. When called in the context of a unit test, I get the following error:

TypeError: You provided 'undefined' where a stream was expected. You can provide an Observable, Promise, Array, or Iterable.

This is a minimal app generated with Angular CLI. Here's the pertinent code:

app.module.ts:

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { HttpClientModule } from '@angular/common/http';
import { AppComponent } from './app.component';

@NgModule({
  declarations: [AppComponent],
  imports: [BrowserModule, HttpClientModule],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

app.component.ts:

import { Component, OnInit } from '@angular/core';
import { TestService } from './test.service'

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css'],
  providers: [TestService]
})
export class AppComponent {
  title = 'app';

  constructor(private testSvc: TestService) {}

  ngOnInit() {
    this.testSvc.executeGet('http://www.example.com');
  }
}

test.service.ts:

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

@Injectable()
export class TestService {
  constructor(private http:HttpClient) {}

  executeGet(url:string) {
    this.http.get(url)
      .subscribe(
        data => console.log('success', data),
        error => console.log('error', error)
    );
  }
}

test.service.spec.ts:

import { HttpClient, HttpHandler } from '@angular/common/http';
import { TestBed, inject } from '@angular/core/testing';
import { TestService } from './test.service';

describe('TestService', () => {
  beforeEach(() => {
    TestBed.configureTestingModule({
      providers: [HttpClient, HttpHandler, TestService]
    });
  });

  it('should executeGet', inject([TestService], (svc: TestService) => {
    expect(svc.executeGet).toBeDefined();
    // this is where the 'undefined' error occurs
    svc.executeGet('http://www.example.com'); 
  }));
});

Any guidance or pointers greatly appreciated.