Root path procider makes a call to Assembly.GetEntryAssembly() which will return NULL under certain circumstances (for example when using NServiceBus host is running as a service), after which Nancy will look for views etc in System32.  I'm not sure WHY this is happening, as one friend said that's some deep voodoo there. :) But one fix is to use Assembly.GetExecutingAssembly() instead of Environment.CurrentDirectory. See https://github.com/btompkins/Nancy.Demo.Hosting.NServiceBus for an example of this breaking (install as service to get it to break)