How do i...mysql C# connection.open problem

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C#

MySQL

I'm creating a Windows Forms App (.NET Framework) and trying to connect to a MySQL Database on my LAN.

The code I'm using is:

private void button1_Click(object sender, EventArgs e)
{
    connString = $"SERVER={server}; DATABASE={database};
                 UID={username}; PASSWORD={password}";
    try{
            conn = new MySqlConnection();
            conn.ConnectionString = connString;
            conn.Open();
            MessageBox.Show("Connected.");
       }
       catch (MySqlException ex)
       {
            MessageBox.Show(ex.Message);
       }
}

When I step through the code and it gets to conn.Open();
I receive the following error:

System.InvalidCastException HResult=0x80004002 Message=Object cannot be cast from DBNull to other types.

However, the database opens and I can Add, Delete and Query the database.

Anyone tell this novice what is going on?

What I have tried:

I've Googled the error and none of it seems to make sense (to me) as
the database connection is fine.