diff --git a/tests/pvec/t_pvec.ml b/tests/pvec/t_pvec.ml
index 2dee7e09..8078577d 100644
--- a/tests/pvec/t_pvec.ml
+++ b/tests/pvec/t_pvec.ml
@@ -265,10 +265,14 @@ module Op = struct
                    ( 1,
                      list_small gen_x >|= fun l ->
                      Append l, size + List.length l );
+                 ];
+                 (if size < 10_000 then [
+                   (* flat map can explode, only do it if list isn't too big *)
                    ( 1,
                      list_size (0 -- 5) gen_x >|= fun l ->
                      Flat_map l, size * (1 + List.length l) );
-                 ];
+                 ] else [])
+                 ;
                ]
         in