listview adapter

I want to set a string array to my list view. With below code I have "An unhandled exception" in line 3. What is problem? Did I forget something?

string[] items;
items = {"01", "02", "03", "04"};
ArrayAdapter<string> adapter = new ArrayAdapter<string>(this, Android.Resource.Layout.SimpleListItem1, items);
lstview.Adapter = adapter;