Vanilla Doom "cannot open" wad

For some reason, whenever I make a single level wad and try to open it in vanilla, I get the message "cannot open [wadname].wad" before the "This version of Doom has been modified" message. The full episode 1 replacement I made works, but when I tried taking out all of the maps but one, it wouldn't work. I even tried putting empty map markers for the other levels, but no luck.

That error should only occur if Doom can't create the file handle. Is the WAD open in another program?

This is the code that produces the error:

    if ( (handle = fopen (filename,"rb")) == NULL)
    {
	printf (" couldn't open %s\n",filename);
	return;
    }

Got it. Filename was too long. Thanks!