I'm using FiddlerCore to build a utility to decode some stuff for me, but I'm hitting an issue with HAR 1.2 import and export. The relevant code is here on GitHub (https://github.com/yaakov-h/TeamRocketProxy/blob/master/src/TeamRocketProxy/TeamRocketProxy/Interception/Http/FiddlerHttpProxy.cs#L70-L101) . If I call Save(), FiddlerCore will create a HAR that I can open with Fiddler4, but not with FiddlerCore (see the Load(string) method). Instead, I get the exception below logged. RunConnectionStateMachine is huge so I can't find any obvious NullReferenceExceptions in ILSpy. I can provide a HAR on request, but I don't want to post it publicly as it contains personal location information, encoded. ******Fiddler ReportException()****** Object reference not set to an instance of an object. at Fiddler.ServerChatter.RunConnectionStateMachine() at Fiddler.Session.RunStateMachine() at Fiddler.Session.Execute(Object objThreadState) IsClosing: False Uncaught Exception in Session #8 - Fiddler has encountered an unexpected problem. If you believe this is a bug in Fiddler, please copy this message by hitting CTRL+C, and submit a bug report using the Help | Send Feedback menu. Object reference not set to an instance of an object. Type: System.NullReferenceException Source: FiddlerCore4 at Fiddler.ServerChatter.RunConnectionStateMachine() at Fiddler.Session.RunStateMachine() at Fiddler.Session.Execute(Object objThreadState) Fiddler v1.0.0.0 (x64 AMD64) [.NET 4.0.30319.42000 on Microsoft Windows NT 6.2.9200.0] Uncaught Exception in Session #8 - Fiddler has encountered an unexpected problem. If you believe this is a bug in Fiddler, please copy this message by hitting CTRL+C, and submit a bug report using the Help | Send Feedback menu. Object reference not set to an instance of an object. Type: System.NullReferenceException Source: FiddlerCore4 at Fiddler.ServerChatter.RunConnectionStateMachine() at Fiddler.Session.RunStateMachine() at Fiddler.Session.Execute(Object objThreadState) Fiddler v1.0.0.0 (x64 AMD64) [.NET 4.0.30319.42000 on Microsoft Windows NT 6.2.9200.0] Uncaught Exception in Session #8 - Fiddler has encountered an unexpected problem. If you believe this is a bug in Fiddler, please copy this message by hitting CTRL+C, and submit a bug report using the Help | Send Feedback menu. Object reference not set to an instance of an object. Type: System.NullReferenceException Source: FiddlerCore4 at Fiddler.ServerChatter.RunConnectionStateMachine() at Fiddler.Session.RunStateMachine() at Fiddler.Session.Execute(Object objThreadState) Fiddler v1.0.0.0 (x64 AMD64) [.NET 4.0.30319.42000 on Microsoft Windows NT 6.2.9200.0] ******Fiddler ReportException()****** Object reference not set to an instance of an object. at Fiddler.ServerChatter.RunConnectionStateMachine() at Fiddler.Session.RunStateMachine() at Fiddler.Session.Execute(Object objThreadState) IsClosing: False Uncaught Exception in Session #9 - Fiddler has encountered an unexpected problem. If you believe this is a bug in Fiddler, please copy this message by hitting CTRL+C, and submit a bug report using the Help | Send Feedback menu. Object reference not set to an instance of an object. Type: System.NullReferenceException Source: FiddlerCore4 at Fiddler.ServerChatter.RunConnectionStateMachine() at Fiddler.Session.RunStateMachine() at Fiddler.Session.Execute(Object objThreadState) Fiddler v1.0.0.0 (x64 AMD64) [.NET 4.0.30319.42000 on Microsoft Windows NT 6.2.9200.0] Uncaught Exception in Session #9 - Fiddler has encountered an unexpected problem. If you believe this is a bug in Fiddler, please copy this message by hitting CTRL+C, and submit a bug report using the Help | Send Feedback menu. Object reference not set to an instance of an object. Type: System.NullReferenceException Source: FiddlerCore4 at Fiddler.ServerChatter.RunConnectionStateMachine() at Fiddler.Session.RunStateMachine() at Fiddler.Session.Execute(Object objThreadState) Fiddler v1.0.0.0 (x64 AMD64) [.NET 4.0.30319.42000 on Microsoft Windows NT 6.2.9200.0] Uncaught Exception in Session #9 - Fiddler has encountered an unexpected problem. If you believe this is a bug in Fiddler, please copy this message by hitting CTRL+C, and submit a bug report using the Help | Send Feedback menu. Object reference not set to an instance of an object. Type: System.NullReferenceException Source: FiddlerCore4 at Fiddler.ServerChatter.RunConnectionStateMachine() at Fiddler.Session.RunStateMachine() at Fiddler.Session.Execute(Object objThreadState) Fiddler v1.0.0.0 (x64 AMD64) [.NET 4.0.30319.42000 on Microsoft Windows NT 6.2.9200.0]