c++ & sfml unhandled exception

I've been following an online tutorial to make a using using the SFML framework with c++. The current build should run however it breaks and I keep getting an unhandled exception error.

Here's the last few lines of my Debug output (There's a whole lot of lines that say: 'SFML city builder.exe' (Win32): Loaded 'C:\Windows\System32\winmm.dll'. Cannot find or open the PDB file.enter code here with various other dlls)

The thread 0xeb0 has exited with code 0 (0x0).
The thread 0x2898 has exited with code 0 (0x0).
The thread 0x1a38 has exited with code 0 (0x0).
The program '[11948] SFML city builder.exe' has exited with code 0 (0x0).

Here is the part that gives the exception: (the arrow)

mapped_type& at(const key_type& _Keyval)
        {   // find element matching _Keyval
        iterator _Where = this->lower_bound(_Keyval);
        if (_Where == this->end()
            || this->_Getcomp()(_Keyval, this->_Key(_Where._Mynode())))
            _Xout_of_range("invalid map<K, T> key");
  ->        return (_Where->second);
            }

Does anyone know what the problem could be? Any help would be much appreciated!

Cheers, Caleb.