'InterstitialAd' is abstract; cannot be instantiated

I'm updating one of my Android apps but I'm finding some problems concerning AdMob implementation (which was working well on previous versions).

This is my code:

public class Edition extends AppCompatActivity {

  private InterstitialAd mInterstitialAd;

  @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_edition);

        mInterstitialAd = new InterstitialAd(this); //ERROR
        mInterstitialAd.setAdUnitId(getString(R.string.full_screen_test_ad));
        mInterstitialAd.loadAd(new AdRequest.Builder().build());
        mInterstitialAd.setAdListener(new AdListener() {
            @Override
            public void onAdClosed() {
                mInterstitialAd.loadAd(new AdRequest.Builder().build());
            }
        });
  }
}

The initialize() method is called during my MainActivity.

But the following error appears on the marked line:

'InterstitialAd' is abstract; cannot be instantiated

Thanks in advance for any comment you can have!