diff --git a/ICSharpCode.NRefactory.CSharp/Completion/CSharpParameterCompletionEngine.cs b/ICSharpCode.NRefactory.CSharp/Completion/CSharpParameterCompletionEngine.cs
index 7ead2cce85..76c266244c 100644
--- a/ICSharpCode.NRefactory.CSharp/Completion/CSharpParameterCompletionEngine.cs
+++ b/ICSharpCode.NRefactory.CSharp/Completion/CSharpParameterCompletionEngine.cs
@@ -173,6 +173,8 @@ namespace ICSharpCode.NRefactory.CSharp.Completion
 					}
 					if (invoke.Node is ObjectCreateExpression) {
 						var createType = ResolveExpression(((ObjectCreateExpression)invoke.Node).Type, invoke.Unit);
+						if (createType.Item1.Type.Kind == TypeKind.Unknown)
+							return null;
 						return factory.CreateConstructorProvider(document.GetOffset(invoke.Node.StartLocation), createType.Item1.Type);
 					}
 				
diff --git a/ICSharpCode.NRefactory.Tests/CSharp/CodeCompletion/CodeCompletionBugTests.cs b/ICSharpCode.NRefactory.Tests/CSharp/CodeCompletion/CodeCompletionBugTests.cs
index 8af7a17498..047b956062 100644
--- a/ICSharpCode.NRefactory.Tests/CSharp/CodeCompletion/CodeCompletionBugTests.cs
+++ b/ICSharpCode.NRefactory.Tests/CSharp/CodeCompletion/CodeCompletionBugTests.cs
@@ -4951,5 +4951,25 @@ $#region S$
 }");
 			Assert.IsTrue(provider == null || provider.Count == 0);
 		}
+
+		/// <summary>
+		/// Bug 4323 - Parameter completion exception while attempting to instantiate unknown class
+		/// </summary>
+		[Test()]
+		public void TestBug4323()
+		{
+			// just test for exception
+			ParameterCompletionTests.CreateProvider(
+@"namespace Test
+{
+    class TestClass
+    {
+        public static void Main(string[] args)
+        {
+            $object foo = new Foo($
+        }
+    }
+}");
+		}
 	}
 }
diff --git a/ICSharpCode.NRefactory.Tests/CSharp/CodeCompletion/ParameterCompletionTests.cs b/ICSharpCode.NRefactory.Tests/CSharp/CodeCompletion/ParameterCompletionTests.cs
index 43bdc4e93f..747cee5f57 100644
--- a/ICSharpCode.NRefactory.Tests/CSharp/CodeCompletion/ParameterCompletionTests.cs
+++ b/ICSharpCode.NRefactory.Tests/CSharp/CodeCompletion/ParameterCompletionTests.cs
@@ -212,9 +212,9 @@ namespace ICSharpCode.NRefactory.CSharp.CodeCompletion
 			}
 			
 			#region IParameterCompletionDataFactory implementation
-			public IParameterDataProvider CreateConstructorProvider (int startOffset, ICSharpCode.NRefactory.TypeSystem.IType type)
+			public IParameterDataProvider CreateConstructorProvider(int startOffset, ICSharpCode.NRefactory.TypeSystem.IType type)
 			{
-				
+				Assert.IsTrue(type.Kind != TypeKind.Unknown);
 				return new Provider () {
 					Data = type.GetConstructors (m => m.Accessibility == Accessibility.Public)
 				};
@@ -227,15 +227,17 @@ namespace ICSharpCode.NRefactory.CSharp.CodeCompletion
 				};
 			}
 
-			public IParameterDataProvider CreateDelegateDataProvider (int startOffset, ICSharpCode.NRefactory.TypeSystem.IType type)
+			public IParameterDataProvider CreateDelegateDataProvider(int startOffset, ICSharpCode.NRefactory.TypeSystem.IType type)
 			{
+				Assert.IsTrue(type.Kind != TypeKind.Unknown);
 				return new Provider () {
 					Data = new [] { type.GetDelegateInvokeMethod () }
 				};
 			}
 			
-			public IParameterDataProvider CreateIndexerParameterDataProvider (int startOffset, IType type, AstNode resolvedNode)
+			public IParameterDataProvider CreateIndexerParameterDataProvider(int startOffset, IType type, AstNode resolvedNode)
 			{
+				Assert.IsTrue(type.Kind != TypeKind.Unknown);
 				if (type.Kind == TypeKind.Array)
 					return new ArrayProvider ();
 				return new IndexerProvider () {