Reader only select empty Fields from MySQL-Database - Error Message: The Value cannot be NULL Paramter: Item

I got another problem. If I try to select the Database from a MySQL Database I get a Error Message: The Value can not be NULL. Paramter: item (In German its: Der Wert darf nicht NULL sein. Parameter: item) So that means the Reader only select empty fields - he "selects NULL" - but the fields arent empty in my SQL-Database.

private: void Fillcombo(void) {

String^ constring = L"datasource=localhost;port=3306;username=root;password=ichstinkenachmaggie";
MySqlConnection^ conDataBase = gcnew MySqlConnection(constring);
MySqlCommand^ cmdDataBase = gcnew MySqlCommand("select rank from database.ranks;", conDataBase);
MySqlDataReader^ myReader;

try {

    conDataBase->Open();
    myReader = cmdDataBase->ExecuteReader();
    while (myReader->Read()) {
        String^ vRank;
        myReader->GetString("rank");
        rank_combo->Items->Add(vRank);

    }

    }
catch (Exception^ex) {

    MessageBox::Show(ex->Message);

}

}

If replace the line:

rank_combo->Items->Add(vRank);

with this:

rank_combo->Items->Add(vRank + "*****");

then I dont get the error but in the combobox there are only 4 lines with 5 ***** :(

Hopefully you can help me somehow :)