diff --git a/library/dialect/swi/fli/swi.c b/library/dialect/swi/fli/swi.c
index 1cbc22477..77e3f75ff 100755
--- a/library/dialect/swi/fli/swi.c
+++ b/library/dialect/swi/fli/swi.c
@@ -2488,7 +2488,7 @@ pl_thread_self(void)
 #if THREADS
   if (pthread_getspecific(Yap_yaamregs_key) == NULL)
     return -1;
-  return worker_id+1;
+  return worker_id;
 #else
   return -2;
 #endif