diff -Nrua old//src/jsgc.c src//src/jsgc.c
--- old//src/jsgc.c	2010-12-13 15:13:22.000000000 +0100
+++ src//src/jsgc.c	2010-12-13 15:15:19.000000000 +0100
@@ -1126,7 +1126,7 @@
     uint32 index;
 
     a = THING_TO_ARENA(thing);
-    if (!a->list)
+    if (!a->list || !a->list->thingSize)
         return JSTRACE_DOUBLE;
 
     index = THING_TO_INDEX(thing, a->list->thingSize);