Confusing error with query in c#

Im having problems with some code ill try to do.Im doing something for suppliers orders , so i have a table wich is named "encomendas_fornecedores" with a autoincrement field before the key that is the code of sale wich consists in a EF before the number(wich is a text field).So heres the code:

connection.Open();

        OleDbCommand comando1 = new OleDbCommand();
        OleDbCommand comando2 = new OleDbCommand();
        OleDbCommand comando3 = new OleDbCommand();
        comando1.Connection = connection;
        comando2.Connection = connection;
        comando3.Connection = connection;
        comando1.CommandText = "INSERT INTO encomendas_fornecedores (cod_encomenda_forn, cod_metodo, cod_forn, total_pagar_forn) VALUES('FO', '" + txtcodmetodo.Text + "', '" + txtcodforn.Text + "', '" + lbltotalapagar.Text + "'); ";// insert into table the values with a FO to cod
        comando1.ExecuteNonQuery();
        comando2.CommandText = "Select MAX(num_encomenda) From encomendas_fornecedores;";// selecting maximum num encomenda so i can isolate it and add to a text before(btw i do this in php/sql no problems
       int  numero = Convert.ToInt32(comando2.ExecuteScalar());//max num_encomenda
        string codencomendaforn= "EF"+Convert.ToString(numero);// sales code completed
        comando3.CommandText = "UPDATE encomendas_fornecedores SET cod_encomenda_forn = '"+codencomendaforn+"' WHERE num_encomenda = '"+ numero +"';";//query that is giving me the problems,it says something like "type of data incorrect in data expression"
        comando3.ExecuteScalar();//giving me error this line

        connection.Close();

But now here's the catch the cod_encomenda_forn is text and the num_encomenda auto increment as it is in the sql,and i tried to show the query in a textbox to see if its anything is wrong but nothing seems wrong