diff --git a/src/index-format/granular_marshal.ml b/src/index-format/granular_marshal.ml
index 247e08925..67dd499fb 100644
--- a/src/index-format/granular_marshal.ml
+++ b/src/index-format/granular_marshal.ml
@@ -68,7 +68,7 @@ let () =
       | Some (_, fd) -> close_in fd)
 
 let force_open_store store =
-  let fd = open_in store.filename in
+  let fd = open_in_bin store.filename in
   last_open_store := Some (store, fd);
   fd