Getting access to std::map value (a struct)

Hello

I am struggling here with a std::map like this:

struct agents
{
  Socket* agentsocket; // ptr to socket for client
  DWORD dwDeviceID; // (DeviceID)
  char szLogon[100]; // logon
};
agents thisagent;
thisagent.agentsocket = 0;
thisagent.dwDeviceID = 6;
lstrcpy(thisagent.szLogon, "Angus");
agents thatagent;
thatagent.agentsocket = 0;
thatagent.dwDeviceID = 8;
lstrcpy(thatagent.szLogon, "Lisa");
// std::map<Key,Val>:
std::map<int, agents> m_AgentsList;
m_AgentsList[4] = thisagent;
m_AgentsList[8] = thatagent;

So if I want to get access to the agents in m_AgentsList[4] I would do this:
agents myagent = m_AgentsList[4]; // compile error line
std::cout << myagent.szLogon << std::endl;
Easy enough. This compiles in my test program. But if I compile in my main
program I get this:
SocketServer.cpp(410) :
error C2678:
binary '[' : no operator defined which takes a left-hand operand of type
'const class std::map<int,struct CSocketServe
r::agents,struct std::less<int>,class std::allocator<struct
CSocketServer::agents> >' (or there is no acceptable conversion)

I have exactly the same code in SocketServer.cpp as my test code in a really
small test cpp file.

Angus