Tengo un procedimiento almacenado que tiene un parámetro sin valor predeterminado, pero puede ser nulo. Pero no sé cómo pasar nulo con Dapper. Puedo hacerlo bien en ADO.

connection.Execute("spLMS_UpdateLMSLCarrier", new { **RouteId = DBNull.Value**, CarrierId = carrierID, UserId = userID, TotalRouteRateCarrierId = totalRouteRateCarrierId },
                                        commandType: CommandType.StoredProcedure);

Excepción:

System.NotSupportedException was caught
  Message=The member RouteId of type System.DBNull cannot be used as a parameter value
  Source=Dapper
  StackTrace:
       at Dapper.SqlMapper.LookupDbType(Type type, String name) in C:\Dev\dapper-git\Dapper\SqlMapper.cs:line 348
       at Dapper.SqlMapper.CreateParamInfoGenerator(Identity identity) in C:\Dev\dapper-git\Dapper\SqlMapper.cs:line 1251
       at Dapper.SqlMapper.GetCacheInfo(Identity identity) in C:\Dev\dapper-git\Dapper\SqlMapper.cs:line 908
       at Dapper.SqlMapper.Execute(IDbConnection cnn, String sql, Object param, IDbTransaction transaction, Nullable`1 commandTimeout, Nullable`1 commandType) in C:\Dev\dapper-git\Dapper\SqlMapper.cs:line 532
       at Rating.Domain.Services.OrderRatingQueueService.UpdateLMSLCarrier(Int32 totalRouteRateCarrierId, Int32 carrierID, Int32 userID) in C:\DevProjects\Component\Main\Source\Rating\Source\Rating.Domain\Services\OrderRatingQueueService.cs:line 52
       at Benchmarking.Program.OrderRatingQueue() in C:\DevProjects\Component\Main\Source\Benchmarking\Source\Benchmarking\Program.cs:line 81
       at Benchmarking.Program.Main(String[] args) in C:\DevProjects\Component\Main\Source\Benchmarking\Source\Benchmarking\Program.cs:line 28
  InnerException: 

No puedo dejar el RouteId apagado, me da un error que dice que es necesario. No se puede utilizar null regular o me da otro error. No se puede cambiar el procedimiento almacenado, no me pertenece.