Should Dapper be able to access a private getter?

I'm working with Dapper and DB2. As far as I can see DB2 doesn't support GUID's natively, so I'm storing it as a string and using dappers access to private properties to help out.

class TestTable
{
    public string RequestId
    {
        get { return RequestGuid.ToString(); }
        private set { RequestGuid = new Guid(value); }
    }

    public Guid RequestGuid { get; set; }
}

I would rather RequestId was fully private, however if I do that the following fails

    connection.Execute("INSERT INTO SOMETABLE (REQUESTID) VALUES(@REQUESTID)"
        , new TestTable() { RequestGuid = Guid.NewGuid() });

The error is

Not enough parameters specified. The command requires 1 parameter(s), but only 0 parameter(s) exist in the parameter collection.

Should dapper be able to access a private getter?

Edit:

The connection.Execute works if I use the class as it is shown. I get the error if I make the RequestId private.