Dapper issue

Ciao Ragazzi,

ho un problema ( forse banale ) ma non ci esco.

Questo spezzone di codice funziona :

       Using conn = Core.Config.Common.GetISBetsConnection()
               toRet = conn.QuerySingleOrDefault(Of IlMioDTO)(sqlStmt, New With
                         {
                           .A = a,
                           .B = b,
                           .C = c
                         })
           End Using

mentre questo va in errore :

           Using conn = Core.Config.Common.GetISBetsConnection()
               toRet = conn.QuerySingleOrDefault(Of IlMioDTO)(sqlStmt, New With
                         {
                           .A = a,
                           .B = b,
                           .C = New Dapper.DbString() With
                                                             {
                                                             .Value = c,
                                                             .IsAnsi = True,
                                                             .Length = 100
                                                             }
                         })
           End Using

 L'errore che ho in fase di compilazione è il seguente :

Error BC30521 Overload resolution failed because no accessible 'QuerySingleOrDefault' is most specific for these arguments:
   Extension method 'Public Function QuerySingleOrDefault(Of IlMioDTO)(sql As String, [param As Object = Nothing], [transaction As IDbTransaction = Nothing], [commandTimeout As Integer? = Nothing], [commandType As CommandType? = Nothing]) As IlMioDTO' defined in 'SqlMapper': Not most specific.
   Extension method 'Public Function QuerySingleOrDefault(Of IlMioDTO)(sql As String, [param As Object = Nothing], [transaction As IDbTransaction = Nothing], [commandTimeout As Integer? = Nothing], [commandType As CommandType? = Nothing], [lineNumber As Integer = 0], [methodName As String = Nothing], [filePath As String = Nothing]) As IlMioDTO' defined in 'SqlMapperExtensions': Not most specific. 

Attendo prezioso aiuto.
Grazie,
Simone