Application hangs at Result call to at()...

[Graham Reitz]

I have an application that does the following:

unsigned int some_db::client_id_query(std::string &client_username,
                            std::string &client_password,
                            std::string &error_msg)
{
	try
	{
         Query client_id_query = m_connection.query();
         client_id_query << "select client_id from clients where  
client_user_name ="
             << quote_only << client_username << " and client_password  
="
             << quote_only << client_password;

         cout << "Query created\n";

         Result client_id_result = client_id_query.store();

         cout << "Result received\n";

         Row client_id_row;
         if (client_id_row = client_id_result.at(0))  // It hangs  
here, and eventually hard crashes.
         {
             return client_id_row.at(0);
         }
     }
     catch (exception &e)
     {
         error_msg = e.what();
     }
     catch (...)
     {
         error_msg = "Unknown exception";
     }
     return 0;
}

The output is:
Connected to the some database (this is from the main function)
Query created
Result received (hangs here for about 20-30 seconds and than hard  
crashes)

I can perform a manual query in the database using the mysql client  
and it returns a result without issue.
What would cause it to hang for about 20-30 seconds at the  
client_id_result.at(0) call?
If there is something fundamentally wrong with this code I would be  
grateful for any advice.

Thanks,
graham