Ironpython和C-脚本访问C对象

考虑下面的代码:

ScriptRuntimeSetup setup = Python.CreateRuntimeSetup(null);
ScriptRuntime runtime = new ScriptRuntime(setup);
ScriptEngine engine = Python.GetEngine(runtime);
ScriptScope scope = engine.CreateScope();

scope.SetVariable("message", "Hello, world!");
string script = @"print message";
ScriptSource source = scope.Engine.CreateScriptSourceFromString(script, SourceCodeKind.Statements);
source.Execute();

此代码产生以下异常:

Microsoft.Scripting.Runtime.UnboundNameException(Microsoft.Scripting.Runtime.UnboundNameException) was unmanaged message=“名称 未定义“message”

我错过了什么?