diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index ef026b2..f3b2f50 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -1300,6 +1300,7 @@
             else
                 m_obj->size = low;
 #endif
+            m_obj->size -= offset;
         } else {
             m_obj->size = map_size;
         }