diff --git a/ptests/ptests.ml b/ptests/ptests.ml
index 64401651909d77b6750644327c810e154d6dce20..2b8a22fe7c625718139bb778908499fce698eb32 100644
--- a/ptests/ptests.ml
+++ b/ptests/ptests.ml
@@ -1363,6 +1363,8 @@ let check_file_is_empty_or_nonexisting diff ~log_file =
     0
   else begin
     lock();
+    (* signal that there's a problem. *)
+    shared.summary_log <- shared.summary_log + 1;
     Queue.push diff shared.diffs;
     Condition.signal shared.diff_available;
     unlock();
@@ -1531,7 +1533,6 @@ let do_diff = function
         let diff_string = diff_check_exist oracle_file result_file in
         ignore (launch diff_string)
 
-
 let diff_thread () =
   lock () ;
   while true do