diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index 6ca95c9..0e91f2c 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -1346,6 +1346,7 @@
             else
                 m_obj->size = low;
 #endif
+            m_obj->size -= offset;
         } else {
             m_obj->size = map_size;
         }