ContentType not supported 'video/mp4'

I have the following exception logged by the UncaughtExceptionHandler from a production site that I am not sure what the problem is or what it’s specifically moaning about:.

We do have a service that streams video files of type mp4 but that works as expected. What would cause this exception and is it something to be concerned about?

SS: 5.1.0

System.NotSupportedException: ContentType not supported 'video/mp4'
   at ServiceStack.Host.ContentTypes.<SerializeUnknownContentType>d__26.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at ServiceStack.HttpResponseExtensionsInternal.<WriteErrorToResponse>d__14.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at ServiceStack.HostContext.<RaiseAndHandleUncaughtException>d__67.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at ServiceStack.HttpResponseExtensionsInternal.<HandleResponseWriteException>d__8.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at ServiceStack.HttpResponseExtensionsInternal.<WriteToResponse>d__7.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at ServiceStack.HttpResponseExtensionsInternal.<WriteToResponse>d__7.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at ServiceStack.Host.Handlers.ServiceStackHandlerBase.<HandleResponseNext>d__13.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at ServiceStack.Host.Handlers.ServiceStackHandlerBase.<HandleResponse>d__12.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at ServiceStack.Host.RestHandler.<ProcessRequestAsync>d__14.MoveNext()

If it’s the same as this previous issue, it’s resolved in the latest v5.1.1 on MyGet. Otherwise I will need a repro to be able to identify what it is.