diff --git a/spot/parsetl/fmterror.cc b/spot/parsetl/fmterror.cc
index e7a231603..a8b262874 100644
--- a/spot/parsetl/fmterror.cc
+++ b/spot/parsetl/fmterror.cc
@@ -89,6 +89,7 @@ namespace spot
           os << '\n' << it.second << "\n\n";
           printed = true;
         }
+      os.flush();
       return printed;
     }
   }