diff --git a/gc.c b/gc.c
index 8802fbdf..0d938bdd 100644
--- a/gc.c
+++ b/gc.c
@@ -501,7 +501,7 @@ sexp_heap sexp_make_heap (size_t size, size_t max_size, size_t chunk_size) {
   sexp_heap h;
 #if SEXP_USE_MMAP_GC
   h =  mmap(NULL, sexp_heap_pad_size(size), PROT_READ|PROT_WRITE|PROT_EXEC,
-            MAP_ANON|MAP_PRIVATE, 0, 0);
+            MAP_ANON|MAP_PRIVATE, -1, 0);
   if (h == MAP_FAILED) return NULL;
 #else
   h =  sexp_malloc(sexp_heap_pad_size(size));