diff --git a/ICSharpCode.Decompiler.Tests/DataFlowTest.cs b/ICSharpCode.Decompiler.Tests/DataFlowTest.cs
index 3c599ccc2..8c49c3949 100644
--- a/ICSharpCode.Decompiler.Tests/DataFlowTest.cs
+++ b/ICSharpCode.Decompiler.Tests/DataFlowTest.cs
@@ -55,10 +55,14 @@ namespace ICSharpCode.Decompiler.Tests
 		public void TryFinallyWithAssignmentInFinally()
 		{
 			ILVariable v = new ILVariable(VariableKind.Local, SpecialType.UnknownType, 0);
-			ILFunction f = new ILFunction((IMethod)null, 0, new GenericContext(), new TryFinally(
-				new Nop(),
-				new StLoc(v, new LdcI4(0))
-			));
+			ILFunction f = new ILFunction(
+				returnType: SpecialType.UnknownType,
+				parameters: new IParameter[0],
+				genericContext: new GenericContext(),
+				body: new TryFinally(
+					new Nop(),
+					new StLoc(v, new LdcI4(0))
+				));
 			f.AddRef();
 			f.Variables.Add(v);
 			f.Body.AcceptVisitor(new RDTest(f, v));
diff --git a/ICSharpCode.Decompiler/IL/Instructions/ILFunction.cs b/ICSharpCode.Decompiler/IL/Instructions/ILFunction.cs
index daaeb214a..d66248133 100644
--- a/ICSharpCode.Decompiler/IL/Instructions/ILFunction.cs
+++ b/ICSharpCode.Decompiler/IL/Instructions/ILFunction.cs
@@ -166,13 +166,11 @@ namespace ICSharpCode.Decompiler.IL
 
 		/// <summary>
 		/// Return type of this function.
-		/// Might be null, if this function was not created from metadata.
 		/// </summary>
 		public readonly IType ReturnType;
 
 		/// <summary>
 		/// List of parameters of this function.
-		/// Might be null, if this function was not created from metadata.
 		/// </summary>
 		public readonly IReadOnlyList<IParameter> Parameters;
 
@@ -188,17 +186,16 @@ namespace ICSharpCode.Decompiler.IL
 		/// </summary>
 		/// <remarks>
 		/// Use <see cref="ILReader"/> to create ILAst.
-		/// <paramref name="method"/> may be null.
 		/// </remarks>
 		public ILFunction(IMethod method, int codeSize, GenericContext genericContext, ILInstruction body, ILFunctionKind kind = ILFunctionKind.TopLevelFunction) : base(OpCode.ILFunction)
 		{
 			this.Method = method;
-			this.Name = Method?.Name;
+			this.Name = method.Name;
 			this.CodeSize = codeSize;
 			this.GenericContext = genericContext;
 			this.Body = body;
-			this.ReturnType = Method?.ReturnType;
-			this.Parameters = Method?.Parameters;
+			this.ReturnType = method.ReturnType;
+			this.Parameters = method.Parameters;
 			this.Variables = new ILVariableCollection(this);
 			this.LocalFunctions = new InstructionCollection<ILFunction>(this, 1);
 			this.kind = kind;
@@ -207,15 +204,15 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// This constructor is only to be used by the TransformExpressionTrees step.
 		/// </summary>
-		internal ILFunction(IType returnType, IReadOnlyList<IParameter> parameters, GenericContext genericContext, ILInstruction body) : base(OpCode.ILFunction)
+		internal ILFunction(IType returnType, IReadOnlyList<IParameter> parameters, GenericContext genericContext, ILInstruction body, ILFunctionKind kind = ILFunctionKind.TopLevelFunction) : base(OpCode.ILFunction)
 		{
 			this.GenericContext = genericContext;
 			this.Body = body;
-			this.ReturnType = returnType;
-			this.Parameters = parameters;
+			this.ReturnType = returnType ?? throw new ArgumentNullException(nameof(returnType));
+			this.Parameters = parameters ?? throw new ArgumentNullException(nameof(parameters));
 			this.Variables = new ILVariableCollection(this);
 			this.LocalFunctions = new InstructionCollection<ILFunction>(this, 1);
-			this.kind = ILFunctionKind.ExpressionTree;
+			this.kind = kind;
 		}
 
 		internal override void CheckInvariant(ILPhase phase)
diff --git a/ICSharpCode.Decompiler/IL/Transforms/TransformExpressionTrees.cs b/ICSharpCode.Decompiler/IL/Transforms/TransformExpressionTrees.cs
index 4ed88e0c7..725bce082 100644
--- a/ICSharpCode.Decompiler/IL/Transforms/TransformExpressionTrees.cs
+++ b/ICSharpCode.Decompiler/IL/Transforms/TransformExpressionTrees.cs
@@ -156,8 +156,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			var container = new BlockContainer();
 			container.AddILRange(instruction);
 			var functionType = instruction.Method.ReturnType.TypeArguments[0];
-			var returnType = functionType.GetDelegateInvokeMethod()?.ReturnType;
-			var function = new ILFunction(returnType, parameterList, context.Function.GenericContext, container);
+			var returnType = functionType.GetDelegateInvokeMethod()?.ReturnType ?? SpecialType.UnknownType;
+			var function = new ILFunction(returnType, parameterList, context.Function.GenericContext, container, ILFunctionKind.ExpressionTree);
 			function.DelegateType = functionType;
 			function.Kind = IsExpressionTree(functionType) ? ILFunctionKind.ExpressionTree : ILFunctionKind.Delegate;
 			function.Variables.AddRange(parameterVariablesList);