diff --git a/C/tracer.c b/C/tracer.c
index 97a35195b..ef7669ce5 100644
--- a/C/tracer.c
+++ b/C/tracer.c
@@ -133,8 +133,8 @@ low_level_trace(yap_low_level_port port, PredEntry *pred, CELL *args)
   char *mname;
   Int arity;
   /*  extern int gc_calls; */
-  static PredEntry *pe;
 
+  LOCK(Yap_heap_regs->low_level_trace_lock);
   sc = Yap_heap_regs;
   vsc_count++;
 #ifdef COMMENTED
@@ -147,51 +147,72 @@ low_level_trace(yap_low_level_port port, PredEntry *pred, CELL *args)
   if (port != enter_pred ||
       !pred ||
       pred->ArityOfPE != 4 ||
-      strcmp(RepAtom(NameOfFunctor(pred->FunctorOfPred))->StrOfAE,"in_between_target_phrases"))
+      strcmp(RepAtom(NameOfFunctor(pred->FunctorOfPred))->StrOfAE,"in_between_target_phrases")) {
+    UNLOCK(Yap_heap_regs->low_level_trace_lock);
     return;
-  if (vsc_count < 1246949400LL)
+  }
+  if (vsc_count < 1246949400LL) {
+    UNLOCK(Yap_heap_regs->low_level_trace_lock);
     return;
+  }
   if (vsc_count == 1246949493LL)
     vsc_xstop = TRUE;
-  if (vsc_count < 5646100000LL)
+  if (vsc_count < 5646100000LL) {
+    UNLOCK(Yap_heap_regs->low_level_trace_lock);
     return;
+  }
   if (vsc_count == 5646100441LL)
     vsc_xstop = TRUE;
   if (vsc_count < 2923351500LL) {
+    UNLOCK(Yap_heap_regs->low_level_trace_lock);
     return;
   }
   if (vsc_count == 123536441LL) vsc_xstop = 1;
   if (vsc_count < 5530257LL) {
+    UNLOCK(Yap_heap_regs->low_level_trace_lock);
     return;
   }
   if (vsc_count == 9414280LL) {
     vsc_xstop = TRUE;
   }
   if (vsc_count < 3399741LL) {
+    UNLOCK(Yap_heap_regs->low_level_trace_lock);
     return;
   }
   if (vsc_count == 51021) {
     printf("Here I go\n");
   }
-  if (vsc_count < 52000) return;
+  if (vsc_count < 52000) {
+    UNLOCK(Yap_heap_regs->low_level_trace_lock);
+    return;
+  }
   if (vsc_count > 52000) exit(0);
+  UNLOCK(Yap_heap_regs->low_level_trace_lock);
   return;
   if (vsc_count == 837074) {
     printf("Here I go\n");
   } 
-  if (gc_calls < 1) return;
- {
-  CELL *env_ptr = ENV;
-  PredEntry *p;
-      while (env_ptr) {
-	PredEntry *pe = EnvPreg(env_ptr[E_CP]);
-	printf("%p->",env_ptr,pe);
-	if (vsc_count == 52LL) printf("\n");
-	if (p == pe) return(TRUE);
-	if (env_ptr != NULL)
-	  env_ptr = (CELL *)(env_ptr[E_E]);
+  if (gc_calls < 1) {
+    UNLOCK(Yap_heap_regs->low_level_trace_lock);
+    return;
+  }
+  {
+    CELL *env_ptr = ENV;
+    PredEntry *p;
+
+    while (env_ptr) {
+      PredEntry *pe = EnvPreg(env_ptr[E_CP]);
+
+      printf("%p->",env_ptr,pe);
+      if (vsc_count == 52LL) printf("\n");
+      if (p == pe) {
+	UNLOCK(Yap_heap_regs->low_level_trace_lock);
+	return;
       }
-	printf("\n");
+      if (env_ptr != NULL)
+	env_ptr = (CELL *)(env_ptr[E_E]);
+      }
+      printf("\n");
  }
 #endif
   fprintf(Yap_stderr,"%lld %x ", vsc_count,ActiveSignals);
@@ -199,10 +220,14 @@ low_level_trace(yap_low_level_port port, PredEntry *pred, CELL *args)
   fprintf(Yap_stderr,"(%d)", worker_id);
 #endif
   /* check_trail_consistency(); */
-  if (pred == NULL) 
+  if (pred == NULL) {
+    UNLOCK(Yap_heap_regs->low_level_trace_lock);
     return;
-  if (pred->ModuleOfPred == 0 && !do_trace_primitives)
+  }
+  if (pred->ModuleOfPred == 0 && !do_trace_primitives) {
+    UNLOCK(Yap_heap_regs->low_level_trace_lock);
     return;
+  }
   switch (port) {
   case enter_pred:
     mname = RepAtom(AtomOfTerm(Yap_Module_Name(pred)))->StrOfAE;
diff --git a/changes-5.1.html b/changes-5.1.html
index 8747f12a3..eaf640d60 100644
--- a/changes-5.1.html
+++ b/changes-5.1.html
@@ -16,6 +16,8 @@
 
 <h2>Yap-5.1.2:</h2>
 <ul>
+<li> FIXED: thread aliases were being cleaned up just after being
+installed (obs Paulo Moura).</li>
 <li> FIXED: rb_partial_map was broken.</li>
 <li> NEW: wdgraph and wundgraph libraries, for weighted graphs.</li>
 </ul>
diff --git a/pl/threads.yap b/pl/threads.yap
index c9d2b29a8..7070ccc7a 100644
--- a/pl/threads.yap
+++ b/pl/threads.yap
@@ -55,8 +55,8 @@ thread_create(Goal, Id, Options) :-
 	'$check_callable'(Goal,G0),
 	'$thread_options'(Options, Aliases, Stack, Trail, System, Detached, G0),
 	'$thread_new_tid'(Id),
-	'$add_thread_aliases'(Aliases, Id),
 	'$clean_db_on_id'(Id),
+	'$add_thread_aliases'(Aliases, Id),
 	'$create_mq'(Id),	
 	'$create_thread'(Goal, Stack, Trail, System, Detached, Id).
 
@@ -163,12 +163,10 @@ current_thread(Tid, Status) :-
 	var(Tid), !,
 	'$cur_threads'(0, Tid, Status).
 current_thread(Tid, Status) :-
-	'$check_thread_alias'(Id0,Tid), !,
+	'$check_thread_alias'(Id0,Tid),
+	integer(Id0), !,
 	'$valid_thread'(Id0),
 	'$thr_status'(Id0, Status).
-current_thread(Tid, Status) :- integer(Tid), !,
-	'$valid_thread'(Tid),
-	'$thr_status'(Tid, Status).
 current_thread(Tid, Status) :-
 	'$do_error'(type_error(integer,Tid),current_thread(Tid, Status)).