=== modified file 'src/font.c'
--- src/font.c	2014-06-10 03:32:36 +0000
+++ src/font.c	2014-06-16 07:55:14 +0000
@@ -158,6 +158,7 @@
   Lisp_Object tail, frame;
   struct font_driver_list *fdl;
 
+  eassert (drv);
   for (fdl = font_driver_list; fdl; fdl = fdl->next)
     if (fdl->driver == drv)
       return true;
@@ -219,6 +220,9 @@
     }
   if (size > 0)
     font->props[FONT_SIZE_INDEX] = make_number (pixelsize);
+  /* NULL means that this font is not allocated by any driver yet,
+     and GC may be called before the driver is assigned (#Bug17771).  */
+  font->driver = NULL;
   return font_object;
 }