diff --git a/extlib/benz/gc.c b/extlib/benz/gc.c
index 0ffc7bcf..4b6ee750 100644
--- a/extlib/benz/gc.c
+++ b/extlib/benz/gc.c
@@ -192,20 +192,18 @@ heap_free(pic_state *pic, void *ap)
 
   bp = (union header *)ap - 1;
 
-  assert(bp->s.size > 1);
-
   for (p = pic->heap->freep; ! (bp > p && bp < p->s.ptr); p = p->s.ptr) {
     if (p >= p->s.ptr && (bp > p || bp < p->s.ptr)) {
       break;
     }
   }
-  if (bp + bp->s.size == p->s.ptr) {
+  if (bp + bp->s.size == p->s.ptr && p->s.ptr->s.size > 0) { /* don't melt base header */
     bp->s.size += p->s.ptr->s.size;
     bp->s.ptr = p->s.ptr->s.ptr;
   } else {
     bp->s.ptr = p->s.ptr;
   }
-  if (p + p->s.size == bp && p->s.size > 1) {
+  if (p + p->s.size == bp) {
     p->s.size += bp->s.size;
     p->s.ptr = bp->s.ptr;
   } else {
@@ -221,21 +219,21 @@ heap_morecore(pic_state *pic)
   struct heap_page *page;
   size_t nu;
 
-  nu = (PIC_HEAP_PAGE_SIZE + sizeof(union header) - 1) / sizeof(union header) + 1;
+  nu = PIC_HEAP_PAGE_SIZE / sizeof(union header);
 
-  up = pic_malloc(pic, (1 + nu + 1) * sizeof(union header));
-  up->s.size = nu + 1;
+  assert(nu >= 2);
+
+  up = pic_malloc(pic, PIC_HEAP_PAGE_SIZE);
+  up->s.size = 0;               /* up is never used for allocation */
   heap_free(pic, up + 1);
 
   np = up + 1;
-  np->s.size = nu;
-  np->s.ptr = up->s.ptr;
-  up->s.size = 1;
-  up->s.ptr = np;
+  np->s.size = nu - 1;
+  heap_free(pic, np + 1);
 
   page = pic_malloc(pic, sizeof(struct heap_page));
   page->basep = up;
-  page->endp = up + nu + 1;
+  page->endp = up + nu;
   page->next = pic->heap->pages;
 
   pic->heap->pages = page;
@@ -716,8 +714,9 @@ gc_sweep_page(pic_state *pic, struct heap_page *page)
   union header *bp, *p, *head = NULL, *tail = NULL;
 
   for (bp = page->basep; ; bp = bp->s.ptr) {
-    for (p = bp + bp->s.size; p != bp->s.ptr; p += p->s.size) {
-      if (p == page->endp) {
+    p = bp + (bp->s.size ? bp->s.size : 1); /* first bp's size is 0, so force advnce */
+    for (; p != bp->s.ptr; p += p->s.size) {
+      if (p < page->basep || page->endp <= p) {
         goto escape;
       }
       if (gc_is_marked(p)) {