diff --git a/pthread_stop_world.c b/pthread_stop_world.c
index fbccc29ab..64013e12a 100644
--- a/pthread_stop_world.c
+++ b/pthread_stop_world.c
@@ -862,9 +862,12 @@ GC_INNER void GC_push_all_stacks(void)
         if (0 == lo) ABORT("GC_push_all_stacks: sp not set!");
         if (p->altstack != NULL && (word)p->altstack <= (word)lo
             && (word)lo <= (word)p->altstack + p->altstack_size) {
-          hi = p->altstack + p->altstack_size;
+#         ifdef STACK_GROWS_UP
+            hi = p->altstack;
+#         else
+            hi = p->altstack + p->altstack_size;
+#         endif
           /* FIXME: Need to scan the normal stack too, but how ? */
-          /* FIXME: Assume stack grows down */
         }
         GC_push_all_stack_sections(lo, hi, traced_stack_sect);
 #       ifdef STACK_GROWS_UP