DELETE C# SqlCommand: Subquery returned more than 1 value

I have an error doing either a DELETE or INSERT using a C# SqlCommand; database is SQL server.

The SQL statement is

DELETE FROM [table_name] 
WHERE id = 'some_id';

which is pretty simple.

id is a column name with datatype varchar(15) that can be NULL; some_id has 15 characters.

First, to verify that only 1 row with that ID exists, this SELECT query was made that returns no errors

SELECT * 
FROM [table_name] 
WHERE id = 'some_id';

returns 1 row

The error on the DELETE is

System.Data.SqlClient.SqlException (0x80131904): Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

at System.Data.SqlClient.SqlCommand.ExecuteNonQuery()

This error can happen when more than 1 row satisfy the criteria of the query; clearly not the case here

What is puzzling is that doing the same DELETE query using the C ODBC API there is no error; so it seems the problem is in the C# code

C# code that calls DELETE:

public int execute(string sql)
{
    SqlConnection conn = new SqlConnection(conn_str);

    try
    {
        conn.Open();

        SqlCommand command = new SqlCommand(sql, conn);
        command.ExecuteNonQuery();
        conn.Close();
    }
    catch (Exception ex)
    {
        Console.WriteLine(ex.ToString());
        return -1;
    }

    return 0;
}

C# code that calls SELECT:

public List<List<string>> select(string sql)
{
    Stopwatch sw = new Stopwatch();
    sw.Start();

    List<List<string>> rows = null;
    SqlConnection conn = new SqlConnection(conn_str);

    try
    {
        conn.Open();

        SqlDataReader reader = null;
        SqlCommand command = new SqlCommand(sql, conn);
        reader = command.ExecuteReader();

        int nbr_cols = reader.FieldCount;
        int nbr_rows = 0;

        rows = new List<List<string>>();

        while (reader.Read())
        {
            nbr_rows++;
            List<string> col = new List<string>();

            for (int idx_col = 0; idx_col < nbr_cols; idx_col++)
            {
                 string str = reader[idx_col].ToString();
                 col.Add(str);
            }

            rows.Add(col);
        }

        reader.Close();
        conn.Close();

        sw.Stop();

        Console.WriteLine("Time to get {0} rows: {1} msec", nbr_rows, sw.ElapsedMilliseconds);
    }
    catch (Exception ex)
    {
        Console.WriteLine(ex.ToString());
    }

    return rows;
}

C++ DELETE code with no error

std::string sql = "DELETE FROM [some_table] WHERE ID='some_id';";
  if (query.exec_direct(sql) < 0)
  {
    std::cout << sql;
  }

where the function exec_direct is here

https://github.com/pedro-vicente/lib_odbc/blob/master/odbc.cc

thanks

EDIT

adding link to DELETE documentation

https://docs.microsoft.com/en-us/sql/t-sql/statements/delete-transact-sql?view=sql-server-2017