diff --git a/ulib/dlmalloc/dlmalloc_port.inc b/ulib/dlmalloc/dlmalloc_port.inc
index b5c7fa2..8ead8fb 100644
--- a/ulib/dlmalloc/dlmalloc_port.inc
+++ b/ulib/dlmalloc/dlmalloc_port.inc
@@ -72,11 +72,11 @@ void *sbrk(ptrdiff_t inc) {
   uint64_t pages = _roundpage(inc);
   uint8_t *maddr = NULL;
   int32_t ret = mman_map(NULL, pages, MMAN_MAP_PF_RW, 0, &maddr);
-  if (ret != E_OK) {
+  if (ret != E_OK || maddr == NULL) {
     return 0;
   }
   string_memset(maddr, 0, pages);
-  _last = (void *)(maddr + inc);
+  _last = (void *)(maddr + pages);
   return maddr;
 }