The following classes could not be instantiated: - com.google.android.gms.plus.PlusOneButton

I am following this tutorial http://ift.tt/1c6JGmI to integrate +1 button in my app.

As soon as I put this code in xml I get errors in GraphicalLayout

<com.google.android.gms.plus.PlusOneButton
xmlns:plus="http://ift.tt/18PEJjX"
android:id="@+id/plus_one_button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
plus:size="standard"
plus:annotation="inline" />

I also tried putting xmlns:plus="http://ift.tt/18PEJjX" in parent RelativeLayout, but the same error.

The following classes could not be instantiated:
- com.google.android.gms.plus.PlusOneButton (Open Class, Show Error Log)
See the Error Log (Window > Show View) for more details.


java.lang.NullPointerException
at com.google.android.gms.plus.PlusOneDummyView$zzb.isValid(    at com.google.android.gms.plus.PlusOneDummyView.zzrJ(    at com.google.android.gms.plus.PlusOneDummyView.<init>(    at com.google.android.gms.plus.internal.zzg.zza(    at com.google.android.gms.plus.PlusOneButton.zzN(    at com.google.android.gms.plus.PlusOneButton.<init>(    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(    at sun.reflect.NativeConstructorAccessorImpl.newInstance(    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(    at java.lang.reflect.Constructor.newInstance(    at com.android.ide.eclipse.adt.internal.editors.layout.ProjectCallback.instantiateClass(ProjectCallback.java:438)
at com.android.ide.eclipse.adt.internal.editors.layout.ProjectCallback.loadView(ProjectCallback.java:190)
at android.view.BridgeInflater.loadCustomView(BridgeInflater.java:207)
at android.view.BridgeInflater.createViewFromTag(BridgeInflater.java:132)
at android.view.LayoutInflater.rInflate_Original(LayoutInflater.java:806)
at android.view.LayoutInflater_Delegate.rInflate(LayoutInflater_Delegate.java:64)
at android.view.LayoutInflater.rInflate(LayoutInflater.java:782)
at android.view.LayoutInflater.inflate(LayoutInflater.java:504)
at android.view.LayoutInflater.inflate(LayoutInflater.java:385)

Help !!