The input string was not in the correct format in the MySQL query

In my code, I make several calls to the database in a row. However, when I try to read the integer value from the database, I get the following error: the input string was not in the correct format. This is my code:

private int getNumberOfProjectsAssigned()
{
    ArrayList staffProjects = new ArrayList();
    int numberOfProjects = 0;
    try
    {
        string strConnection = ConfigurationSettings.AppSettings["ConnectionString"];
        MySqlConnection connection = new MySqlConnection(strConnection);
        MySqlCommand command = connection.CreateCommand();
        command.CommandText = "SELECT id_project_pk FROM `test`.`staff_on_project` WHERE id_staff_pk = " + Convert.ToInt32(Session["CurrentUserID"]);
        //SELECT idusers, first_name, last_name, job_title, code_quality, time_bonus,analysis_of_requirements FROM `test`.`users` WHERE security_level > 1;
        connection.Open();

        MySqlDataReader reader = command.ExecuteReader();

        while (reader.Read())
        {
            int projectId = Convert.ToInt32(reader["id_project_pk"].ToString());
            staffProjects.Add(projectId);
        }
        connection.Close();
        foreach (int i in staffProjects)
        {
            command.CommandText = "SELECT still_active FROM `test`.`projects` WHERE idprojects = " + i;
            //SELECT idusers, first_name, last_name, job_title, code_quality, time_bonus,analysis_of_requirements FROM `test`.`users` WHERE security_level > 1;
            connection.Open();

            reader = command.ExecuteReader();

            while (reader.Read())
            {
                int projectId = Convert.ToInt32(reader["still_active"].ToString()); // Error Occurs Here
                if(projectId == 1)
                {
                    projectsStaffWorksOn.Add(projectId);
                }
                numberOfProjects = projectsStaffWorksOn.Count;
            }
            connection.Close();
        }            
    }
    catch { }
    return numberOfProjects;
}

It throws an error at the point marked in the code. Any help would be greatly appreciated!