InflateException em aplicação simples

package com.example.comprassimples;

import android.os.Bundle;
import android.app.Activity;
import android.app.AlertDialog;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.CheckBox;

public class MainActivity extends Activity {

	CheckBox chkarroz, chkfeijao, chkleite, chkcarne;
		
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		
		chkarroz  = (CheckBox) findViewById(R.id.chkarroz);
		chkfeijao = (CheckBox) findViewById(R.id.chkfeijao);
		chkleite  = (CheckBox) findViewById(R.id.chkleite);
		chkcarne  = (CheckBox) findViewById(R.id.chkcarne);
		
		Button btnlocal = (Button) findViewById(R.id.btncompra);
		
		btnlocal.setOnClickListener(new View.OnClickListener()
		{
			@Override
			public void onClick(View v)
			{
				double total = 0;
				
				if(chkarroz.isChecked())
					total += 2.69;
				if(chkfeijao.isChecked())
					total += 5.00;
				if(chkleite.isChecked())
					total += 9.70;
				if(chkcarne.isChecked())
					total += 2.30;
				
				AlertDialog.Builder dialogo = new 
						AlertDialog.Builder(MainActivity.this);
				dialogo.setTitle("Compras");
				dialogo.setMessage("Valor total da compra: "+ String.valueOf(total));
				dialogo.setNeutralButton("ok",null);
				
				dialogo.show();
				
			}
		});
		
	}

	@Override
	public boolean onCreateOptionsMenu(Menu menu) {
		// Inflate the menu; this adds items to the action bar if it is present.
		getMenuInflater().inflate(R.menu.activity_main, menu);
		return true;
	}

}

02-24 21:09:55.725: E/AndroidRuntime(884): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.comprassimples/com.example.comprassimples.MainActivity}: android.view.InflateException: Binary XML file line #7: Error inflating class <unknown>