diff --git a/pl/threads.yap b/pl/threads.yap
index b50fbf9df..b0f45a05e 100644
--- a/pl/threads.yap
+++ b/pl/threads.yap
@@ -39,7 +39,6 @@ volatile(P) :-
 
 '$do_volatile'(P,M) :- dynamic(M:P).
 
-
 :- initialization('$init_thread0').
 
 '$init_thread0' :-
@@ -51,7 +50,7 @@ volatile(P) :-
 	recorda('$thread_defaults', [0, 0, 0, false, true], _),
 	'$create_thread_mq'(0),
 	'$new_mutex'(Id),
-	assert('$with_mutex_mutex'(Id)).
+	assert_static(prolog:'$with_mutex_mutex'(Id)).
 
 '$top_thread_goal'(G, Detached) :-
 	'$thread_self'(Id),
@@ -503,6 +502,9 @@ mutex_unlock_all :-
 	'$unlock_mutex'(Id),
 	'$mutex_unlock_all'(Id).
 
+with_mutex(M, G) :-
+	'$no_threads', !,
+	call(G).
 with_mutex(M, G) :-
 	var(M), !,
 	'$do_error'(instantiation_error,with_mutex(M, G)).