diff --git a/os/pl-prologflag.c b/os/pl-prologflag.c
index d32c444a6..25145dd7d 100755
--- a/os/pl-prologflag.c
+++ b/os/pl-prologflag.c
@@ -298,8 +298,10 @@ setUnknown(term_t value, atom_t a, Module m)
 		      ATOM_modify, ATOM_flag, key);
     }
 
+#ifndef __YAP_PROLOG__
     if ( !SYSTEM_MODE )
       printMessage(ATOM_warning, PL_CHARS, "unknown_in_module_user");
+#endif
   }
 
   m->flags = flags;