Index: src/natools-s_expressions-parsers.adb
==================================================================
--- src/natools-s_expressions-parsers.adb
+++ src/natools-s_expressions-parsers.adb
@@ -102,11 +102,12 @@
       Event : out Events.Event)
    is
       O : Octet;
    begin
       if Self.Locked then
-         raise Constraint_Error;
+         Event := Evnets.End_Of_Input;
+         return;
       end if;
 
       Self.Latest := Events.Error;
       loop
          --  Process pending events