VS2012 - Code Analysis C6255 for conversion from CString to const char *

Hello,

#include <atlstr.h>

int main()
{
   CString str = _T("whatever");
   USES_CONVERSION;
   const char * cnstChar = T2A((LPCTSTR)str);
}

Code Analysis with 'Microsoft All Rules' generates next warning:
source.cpp(7): warning : C6255: _alloca indicates failure by raising a stack overflow exception.  Consider using _malloca instead.

As it's on unicode, T2A is a define for W2A.
And W2A is defined in atlconv.h as:

#define W2A(lpw) (\
	((_lpw = lpw) == NULL) ? NULL : (\
		(_convert = (static_cast<int>(wcslen(_lpw))+1), \
		(_convert>INT_MAX/2) ? NULL : \
		ATLW2AHELPER((LPSTR) alloca(_convert*sizeof(WCHAR)), _lpw, _convert*sizeof(WCHAR), _acp))))

So how to avoid that warning as it comes from a system header? Is there a better way to convert CString? If yes, why is my case bad?

Best regards,