Possible NullReferenceException

            var stages = from stage in doc.Descendants("Stage")
                         where (stage.Attribute("Title") != null &&
                                stage.Attribute("Title").Value.EndsWith(DtwConfig.RequireParameter(DtwConfig.Values.TestStageSpecifier),
                                                                        StringComparison.OrdinalIgnoreCase))


I get "Possible NullReferenceException" on this code on "stage.Attribute("Title").Value.EndsWith". Any idea why it happens?

Version info:

JetBrains ReSharper 5 Full Edition Pre-Release
Build 5.0.1618.49 on 2010-02-17T17:49:36