diff --git a/ICSharpCode.NRefactory/CSharp/Ast/AstNode.cs b/ICSharpCode.NRefactory/CSharp/Ast/AstNode.cs
index f05be52cfd..71e3b5d358 100644
--- a/ICSharpCode.NRefactory/CSharp/Ast/AstNode.cs
+++ b/ICSharpCode.NRefactory/CSharp/Ast/AstNode.cs
@@ -448,6 +448,8 @@ namespace ICSharpCode.NRefactory.CSharp
 		{
 			if (annotation == null)
 				throw new ArgumentNullException("annotation");
+			if (this.IsNull)
+				throw new InvalidOperationException("Cannot add annotations to the null node");
 		retry: // Retry until successful
 			object oldAnnotation = Interlocked.CompareExchange(ref this.annotations, annotation, null);
 			if (oldAnnotation == null) {
diff --git a/ICSharpCode.NRefactory/CSharp/OutputVisitor/IOutputFormatter.cs b/ICSharpCode.NRefactory/CSharp/OutputVisitor/IOutputFormatter.cs
index 3aff02b5ff..ac1966296c 100644
--- a/ICSharpCode.NRefactory/CSharp/OutputVisitor/IOutputFormatter.cs
+++ b/ICSharpCode.NRefactory/CSharp/OutputVisitor/IOutputFormatter.cs
@@ -10,6 +10,9 @@ namespace ICSharpCode.NRefactory.CSharp
 	/// </summary>
 	public interface IOutputFormatter
 	{
+		void StartNode(AstNode node);
+		void EndNode(AstNode node);
+		
 		/// <summary>
 		/// Writes an identifier.
 		/// If the identifier conflicts with a keyword, the output visitor will
diff --git a/ICSharpCode.NRefactory/CSharp/OutputVisitor/OutputVisitor.cs b/ICSharpCode.NRefactory/CSharp/OutputVisitor/OutputVisitor.cs
index 0e487ea3f6..12fea7d59e 100644
--- a/ICSharpCode.NRefactory/CSharp/OutputVisitor/OutputVisitor.cs
+++ b/ICSharpCode.NRefactory/CSharp/OutputVisitor/OutputVisitor.cs
@@ -67,6 +67,7 @@ namespace ICSharpCode.NRefactory.CSharp
 				WriteSpecialsUpToNode(node);
 			currentContainerNode = node;
 			positionStack.Push(node.FirstChild);
+			formatter.StartNode(node);
 		}
 		
 		object EndNode(AstNode node)
@@ -76,6 +77,7 @@ namespace ICSharpCode.NRefactory.CSharp
 			Debug.Assert(pos == null || pos.Parent == node);
 			WriteSpecials(pos, null);
 			currentContainerNode = node.Parent;
+			formatter.EndNode(node);
 			return null;
 		}
 		#endregion
diff --git a/ICSharpCode.NRefactory/CSharp/OutputVisitor/TextWriterOutputFormatter.cs b/ICSharpCode.NRefactory/CSharp/OutputVisitor/TextWriterOutputFormatter.cs
index e14d1032c3..f8c1340e58 100644
--- a/ICSharpCode.NRefactory/CSharp/OutputVisitor/TextWriterOutputFormatter.cs
+++ b/ICSharpCode.NRefactory/CSharp/OutputVisitor/TextWriterOutputFormatter.cs
@@ -107,5 +107,13 @@ namespace ICSharpCode.NRefactory.CSharp
 					break;
 			}
 		}
+		
+		public virtual void StartNode(AstNode node)
+		{
+		}
+		
+		public virtual void EndNode(AstNode node)
+		{
+		}
 	}
 }