[Mono-dev] Error passing DBNull to SQLClient AddParameter method

[APS]

I'm trying to update a record in SQLServer using 
System.Data.SqlClient.SqlCommand:ExecuteNonQuery() method.
Adding parameters to the command I use
IDbDataParameter param=CreateParameter();
param.ParameterName=parameterName;
param.Value=DBNull.Value;

Running the query I obtain the error below

System.Data.SqlClient.SqlException: Disallowed implicit conversion 
from data type sql_variant to data type varchar, table 'tablename', 
column 'columnname'. Use the CONVERT function to run this query.
in <0x000a0> System.Data.SqlClient.SqlConnection:ErrorHandler 
(System.Object sender, 
Mono.Data.Tds.Protocol.TdsInternalErrorMessageEventArgs e)
in (wrapper delegate-invoke) 
System.MulticastDelegate:invoke_void_object_TdsInternalErrorMessageEventArgs 
(object,Mono.Data.Tds.Protocol.TdsInternalErrorMessageEventArgs)
in <0x0001d> Mono.Data.Tds.Protocol.Tds:OnTdsErrorMessage 
(Mono.Data.Tds.Protocol.TdsInternalErrorMessageEventArgs e)
in <0x001bf> Mono.Data.Tds.Protocol.Tds:ProcessMessage 
(TdsPacketSubType subType)
in <0x0012c> Mono.Data.Tds.Protocol.Tds:ProcessSubPacket ()
in <0x00090> Mono.Data.Tds.Protocol.Tds:NextResult ()
in (wrapper remoting-invoke-with-check) 
Mono.Data.Tds.Protocol.Tds:NextResult ()
in <0x00010> Mono.Data.Tds.Protocol.Tds:SkipToEnd ()
in (wrapper remoting-invoke-with-check) Mono.Data.Tds.Protocol.Tds:SkipToEnd ()
in <0x00074> Mono.Data.Tds.Protocol.Tds:ExecuteQuery (System.String 
sql, Int32 timeout, Boolean wantResults)
in <0x0006e> Mono.Data.Tds.Protocol.Tds70:Execute (System.String 
commandText, Mono.Data.Tds.TdsMetaParameterCollection parameters, 
Int32 timeout, Boolean wantResults)
in <0x002eb> System.Data.SqlClient.SqlCommand:Execute 
(CommandBehavior behavior, Boolean wantResults)
in <0x00048> System.Data.SqlClient.SqlCommand:ExecuteNonQuery ()

The same thing i .NET works correctly, I'm doing something's wrong?.
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