diff --git a/mozilla/js/src/jsarena.c b/mozilla/js/src/jsarena.c
index 57be78c432f..9242f3c76ad 100644
--- a/mozilla/js/src/jsarena.c
+++ b/mozilla/js/src/jsarena.c
@@ -204,7 +204,7 @@ JS_ArenaRelease(JSArenaPool *pool, char *mark)
     JSArena *a;
 
     for (a = pool->first.next; a; a = a->next) {
-	if (JS_UPTRDIFF(mark, a) < JS_UPTRDIFF(a->avail, a)) {
+	if (JS_UPTRDIFF(mark, a->base) < JS_UPTRDIFF(a->avail, a->base)) {
 	    a->avail = (jsuword)JS_ARENA_ALIGN(pool, mark);
 	    FreeArenaList(pool, a, JS_TRUE);
 	    return;
diff --git a/mozilla/js/src/jsarena.h b/mozilla/js/src/jsarena.h
index d17fc84a2af..592b9c9d2e3 100644
--- a/mozilla/js/src/jsarena.h
+++ b/mozilla/js/src/jsarena.h
@@ -152,7 +152,7 @@ struct JSArenaPool {
     JS_BEGIN_MACRO                                                            \
 	char *_m = (char *)(mark);                                            \
 	JSArena *_a = (pool)->current;                                        \
-	if (JS_UPTRDIFF(_m, _a) <= JS_UPTRDIFF(_a->avail, _a)) {              \
+	if (JS_UPTRDIFF(_m, _a->base) <= JS_UPTRDIFF(_a->avail, _a->base)) {  \
 	    _a->avail = (jsuword)JS_ARENA_ALIGN(pool, _m);                    \
 	    JS_CLEAR_UNUSED(_a);                                              \
 	    JS_ArenaCountRetract(pool, _m);                                   \