Error Android studio Firebase com.google.android.gms.ads.internal.client.zzaf.zzN

Hi I have this problem:

06-26 18:41:29.996 25912-25912/com.kiry32.root.quizaia E/AndroidRuntime: FATAL EXCEPTION: main java.lang.IllegalStateException: The ad unit ID must be set on InterstitialAd before show is called. at com.google.android.gms.ads.internal.client.zzaf.zzN(Unknown Source) at com.google.android.gms.ads.internal.client.zzaf.show(Unknown Source) at com.google.android.gms.ads.InterstitialAd.show(Unknown Source) at com.kiry32.root.quizaia.MainActivity$5.onClick(MainActivity.java:79) at android.view.View.performClick(View.java:4475) at android.view.View$PerformClick.run(View.java:18786) at android.os.Handler.handleCallback(Handler.java:730) at android.os.Handler.dispatchMessage(Handler.java:92) at android.os.Looper.loop(Looper.java:176) at android.app.ActivityThread.main(ActivityThread.java:5419) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:525) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1046) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:862) at dalvik.system.NativeStart.main(Native Method)`

This is my Gradle:

dependencies {
    compile fileTree(include: ['*.jar'], dir: 'libs')
    testCompile 'junit:junit:4.12'
    compile 'com.android.support:appcompat-v7:23.4.0'
    compile 'com.android.support:design:23.4.0'
    compile 'com.google.firebase:firebase-ads:9.0.2'
}


apply plugin: 'com.google.gms.google-services'

classpath 'com.google.gms:google-services:3.0.0'

Sorry for my English XD