diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index b170b38..c39715a 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -845,7 +845,7 @@
                             "in range(0, 256)");
             return -1;
         }
-        self->data[i] = v;
+        self->data[i] = (char) v;
         return 0;
     }
     else if (PySlice_Check(item)) {