'CMap' of 'CString' to 'CString'

Hello!

I'm trying to create a dictionary with both key and value of type
'CString'. I declare the object of 'CMap' so it is as closer to
std::map, as possible:

CMap<CString, const CString&, CString, const CString&> m_appLangs;

and semantically it's like this:

std::map<std::string, std::string> m_appLangs;

But it seems 'CMap' is not realized as an RB-tree, I guess it must be
realized through hash table. The problem is when I make the above
declaration, my source file refuse to compile. Here is the simplified
error message:

cannot convert from 'const CString' to 'DWORD_PTR'
see reference to function template instantiation 'UINT
HashKey<ARG_KEY>(ARG_KEY)' being compiled with
[ARG_KEY=const CString &]

Here is the code of 'HashKey' function from "afxtempl.h":

template<class ARG_KEY>
AFX_INLINE UINT AFXAPI HashKey(ARG_KEY key)
{
    // default identity hash - works for most primitive values
    return (DWORD)(((DWORD_PTR)key)>>4);
}

And what does it mean? Can't I create a map of string to string just
because there is no conversion from 'CString' to 'DWORD_PTR'? I think/
hope I'm wrong. Would you please show me my mistake?

Thanks in advance
Martin