diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index 8a22752..ef026b2 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -1116,7 +1116,7 @@
 #  endif
     if (fd != -1 && fstat(fd, &st) == 0 && S_ISREG(st.st_mode)) {
         if (map_size == 0) {
-            map_size = st.st_size;
+            map_size = st.st_size - offset;
         } else if ((size_t)offset + (size_t)map_size > st.st_size) {
             PyErr_SetString(PyExc_ValueError,
                             "mmap length is greater than file size");