I have a function defined inside an Oracle package:

CREATE OR REPLACE PACKAGE BODY TESTUSER.TESTPKG as
  FUNCTION testfunc(n IN NUMBER) RETURN NUMBER as
  begin
    return n + 1;
  end testfunc;
end testpkg;
/

How can I call it from C# using Odbc? I tried the following:

using System;
using System.Data;
using System.Data.Odbc;

class Program {
    static void Main(string[] args) {
        using (OdbcConnection connection = new OdbcConnection("DSN=testdb;UID=testuser;PWD=testpwd")) {
            connection.Open();

            OdbcCommand command = new OdbcCommand("TESTUSER.TESTPKG.testfunc", connection);
            command.CommandType = System.Data.CommandType.StoredProcedure;

            command.Parameters.Add("ret", OdbcType.Int).Direction = ParameterDirection.ReturnValue;

            command.Parameters.Add("n", OdbcType.Int).Direction = ParameterDirection.Input;
            command.Parameters["n"].Value = 42;

            command.ExecuteNonQuery();
            Console.WriteLine(command.Parameters["ret"].Value);
        }
    }
}

But I get an exception saying "Invalid SQL Statement".
What am I doing wrong?