java.lang.NoClassDefFoundError

Any idea how fix this error?

 java.lang.NoClassDefFoundError: 
   at com.facebook.appevents.internal.SourceApplicationInfo$Factory.create (SourceApplicationInfo.java:131)
   at com.facebook.appevents.internal.ActivityLifecycleTracker.onActivityCreated (ActivityLifecycleTracker.java:131)
   at com.facebook.appevents.internal.ActivityLifecycleTracker$1.onActivityCreated (ActivityLifecycleTracker.java:77)
   at android.app.Application.dispatchActivityCreated (Application.java:214)
   at android.app.Activity.onCreate (Activity.java:1110)
   at com.unity3d.player.UnityPlayerActivity.onCreate (Unknown Source:4)
   at android.app.Activity.performCreate (Activity.java:7373)
   at android.app.Instrumentation.callActivityOnCreate (Instrumentation.java:1218)
   at android.app.ActivityThread.performLaunchActivity (ActivityThread.java:3137)
   at android.app.ActivityThread.handleLaunchActivity (ActivityThread.java:3292)
   at android.app.ActivityThread.-wrap12 (Unknown Source)
   at android.app.ActivityThread$H.handleMessage (ActivityThread.java:1881)
   at android.os.Handler.dispatchMessage (Handler.java:108)
   at android.os.Looper.loop (Looper.java:166)
   at android.app.ActivityThread.main (ActivityThread.java:7415)
   at java.lang.reflect.Method.invoke (Native Method)
   at com.android.internal.os.Zygote$MethodAndArgsCaller.run (Zygote.java:245)
   at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:921)
 Caused by: java.lang.ClassNotFoundException: 
   at dalvik.system.BaseDexClassLoader.findClass (BaseDexClassLoader.java:93)
   at java.lang.ClassLoader.loadClass (ClassLoader.java:379)
   at java.lang.ClassLoader.loadClass (ClassLoader.java:312)