Problem with RegQueryValueEx

I'm having a problem with the RegQueryValueEx function - it's returning error code 87 (Invalid Parameter), and it's due to the 5th parameter.  I know this, because it returns ERROR_SUCCESS if I make this parameter NULL.

      CString sPwd;
      HKEY      phk;
      char      sPwdReg[10];
      DWORD      dwType;
      DWORD      dwcbData;

//#ifdef WIN32_EXTRA_LEAN
//      this_statement_will_generate_compile_error;
//#endif
      if (::RegOpenKeyEx(HKEY_CURRENT_USER, "Software\\a\\b",
                  0, KEY_ALL_ACCESS, &phk) != ERROR_SUCCESS)
            return;
      AfxMessageBox("OpenKey worked");

      long lRet = ::RegQueryValueEx(phk, (LPTSTR) "Password", NULL, NULL,
                  (LPBYTE) sPwdReg, &dwcbData);

      AfxMessageBox("QueryValue worked");

      CString msg;
      msg.Format("lRet = %ld", lRet);
      AfxMessageBox(msg);

The "Password" registry value is a string, by the way.  Does anybody know what I'm doing wrong here?  I've looked through all the examples in the Visual C++ documentation, and found no clues.  I've seen this argument typecast to LPCTSTR or LPTSTR, but this won't compile.  I've also tried using a char* argument, and allocating memory to it with malloc, but this shouldn't make a difference, and it didn't.

A possibly related problem is that in my Project Settings/C/C++ Tab/Preprocessor Category, I have undefined WIN32_EXTRA_LEAN in order for the compiler to include "winreg.h", which is needed for the registry API.  However, if I uncomment the #ifdef WIN32_EXTRA_LEAN directive, I find that WIN32_EXTRA_LEAN *is* defined.  I cannot figure this out - I'm working around it by just explicitly including "winreg.h".