Error trying to write out mission lighting cache file

I have everything turned on as far as $Prefs:: are concerned to create mission lighting cache files and in the code below in resManager.cpp it returns false where you see ***RETURNS FALSE HERE*** and therefore does not create the .ml file and comes back with this error in sgSceneLighting.cc: "SceneLighting::light: unable to persist lighting!"

The place where it returns false has the comment "don't allow storing files in root" which I assume means in the main directory. My mission file is in [MainDirectory]/Data/Missions/MyMissionFile.mis so what could be causing it to return false here?

bool ResManager::openFileForWrite (FileStream & stream, const char *fileName, U32 accessMode)
{
	if (!isValidWriteFileName (fileName))
      return false;

   // tag it on to the first directory
   char path[1024];
   dStrcpy (path, fileName);
   char *file = dStrrchr (path, '/');
   if (!file)
      return false;      // don't allow storing files in root [b]***RETURNS FALSE HERE***[/b]

   *file++ = 0;

   if (!Platform::createPath (fileName))   // create directory tree
      return false;

   if (!stream.open (fileName, (FileStream::AccessMode) accessMode))
      return false;


   // create a resource for the file.
   ResourceObject *ro = createResource (StringTable->insert (path), StringTable->insert (file));
   ro->flags = ResourceObject::File;
   ro->fileOffset = 0;
   ro->fileSize = 0;
   ro->compressedFileSize = 0;
   return true;
}