diff --git a/core/posix.hh b/core/posix.hh
index f020127ee9..e28fcd7a89 100644
--- a/core/posix.hh
+++ b/core/posix.hh
@@ -228,7 +228,7 @@ public:
 
     void *map(size_t size, unsigned flags, bool shared, size_t offset) {
         void *x = mmap(NULL, size, flags, shared ? MAP_SHARED : MAP_PRIVATE, _fd, offset);
-        throw_system_error_on(x == nullptr);
+        throw_system_error_on(x == MAP_FAILED);
         return x;
     }