diff --git a/C/eval.c b/C/eval.c
index 1f4caf178..74cc498b8 100644
--- a/C/eval.c
+++ b/C/eval.c
@@ -108,6 +108,12 @@ Eval(Term t, E_ARGS)
       }
     }
   } else if (IsPairTerm(t)) {
+    if (TailOfTerm(t) != TermNil) {
+      Yap_Error(TYPE_ERROR_EVALUABLE, t,
+		"string must contain a single character to be evaluated as an arithmetic expression");
+      P = (yamop *)FAILCODE;
+      RERROR();
+    }
     return(Eval(HeadOfTerm(t), USE_E_ARGS));
   } else if (IsIntTerm(t)) {
     RINT(IntOfTerm(t));
@@ -182,6 +188,12 @@ Yap_Eval(Term t, E_ARGS)
       }
     }
   } else if (IsPairTerm(t)) {
+    if (TailOfTerm(t) != TermNil) {
+      Yap_Error(TYPE_ERROR_EVALUABLE, t,
+		"string must contain a single character to be evaluated as an arithmetic expression");
+      P = (yamop *)FAILCODE;
+      RERROR();
+    }
     return(Eval(HeadOfTerm(t), USE_E_ARGS));
   } else if (IsIntTerm(t)) {
     RINT(IntOfTerm(t));
diff --git a/C/threads.c b/C/threads.c
index 6cfb94698..d27f98ff7 100644
--- a/C/threads.c
+++ b/C/threads.c
@@ -101,7 +101,6 @@ kill_thread_engine (int wid)
   Yap_KillStacks(wid);
   Yap_FreeCodeSpace((ADDR)(ThreadHandle[wid].tgoal));
   Yap_heap_regs->wl[wid].active_signals = 0L;
-  pthread_mutex_lock(&(ThreadHandle[wid].tlock));
   free(Yap_heap_regs->wl[wid].scratchpad.ptr);
   free(ThreadHandle[wid].default_yaam_regs);
   ThreadHandle[wid].current_yaam_regs = NULL;
@@ -356,7 +355,6 @@ Yap_thread_detach_engine(int wid)
 Int
 Yap_thread_destroy_engine(int wid)
 {
-  pthread_mutex_lock(&(ThreadHandle[wid].tlock));
   if (ThreadHandle[wid].ref_count == 0) {
     kill_thread_engine(wid);
     return TRUE;
@@ -382,6 +380,7 @@ p_thread_join(void)
     UNLOCK(ThreadHandlesLock);
     return FALSE;
   }
+  pthread_mutex_lock(&(ThreadHandle[tid].tlock));
   UNLOCK(ThreadHandlesLock);
   /* make sure this lock is accessible */
   if (pthread_join(ThreadHandle[tid].handle, NULL) < 0) {
@@ -413,7 +412,6 @@ p_thread_detach(void)
   }
   ThreadHandle[tid].tdetach = 
     MkAtomTerm(AtomTrue);
-  pthread_mutex_unlock(&(ThreadHandle[tid].tlock));
   return TRUE;
 }
 
@@ -670,9 +668,10 @@ p_thread_runtime(void)
 }
 
 static Int 
-p_thread_self_lock(void)
+p_thread_unlock(void)
 {				/* '$thread_self_lock'	 */
-  pthread_mutex_lock(&(ThreadHandle[worker_id].tlock));
+  Int wid = IntegerOfTerm(Deref(ARG1));
+  pthread_mutex_unlock(&(ThreadHandle[wid].tlock));
   return TRUE;
 }
 
@@ -708,6 +707,7 @@ void Yap_InitThreadPreds(void)
   Yap_InitCPred("$nof_threads_created", 1, p_nof_threads_created, SafePredFlag|HiddenPredFlag);
   Yap_InitCPred("$thread_sleep", 4, p_thread_sleep, SafePredFlag|HiddenPredFlag);
   Yap_InitCPred("$thread_runtime", 1, p_thread_runtime, SafePredFlag|HiddenPredFlag);
+  Yap_InitCPred("$thread_unlock", 1, p_thread_unlock, SafePredFlag);
 }
 
 #else
@@ -749,7 +749,7 @@ p_thread_self(void)
 }
 
 static Int 
-p_thread_self_lock(void)
+p_thread_unlock(void)
 {				/* '$thread_runtime'(+P)	 */
   return TRUE;
 }
@@ -762,7 +762,7 @@ void Yap_InitThreadPreds(void)
   Yap_InitCPred("$nof_threads", 1, p_nof_threads, SafePredFlag|HiddenPredFlag);
   Yap_InitCPred("$nof_threads_created", 1, p_nof_threads_created, SafePredFlag|HiddenPredFlag);
   Yap_InitCPred("$thread_runtime", 1, p_thread_runtime, SafePredFlag|HiddenPredFlag);
-  Yap_InitCPred("$self_thread_lock", 0, p_thread_self_lock, SafePredFlag);
+  Yap_InitCPred("$thread_unlock", 1, p_thread_unlock, SafePredFlag);
 }
 
 
diff --git a/docs/yap.tex b/docs/yap.tex
index 054eed82e..d25f39499 100644
--- a/docs/yap.tex
+++ b/docs/yap.tex
@@ -3445,7 +3445,8 @@ the call.
         @var{Char} is a lower-case letter.
 
 @item    lower(Upper)
-        @var{Char} is a lower-case version of Upper. Only true if @var{Char} is lowercase and Upper uppercase.
+        @var{Char} is a lower-case version of @var{Upper}. Only true if
+        @var{Char} is lowercase and @var{Upper} uppercase.
 
 @item    to_lower(Upper)
         @var{Char} is a lower-case version of Upper. For non-letters, or letter without case, @var{Char} and Lower are the same. See also upcase_atom/2 and downcase_atom/2.
diff --git a/pl/threads.yap b/pl/threads.yap
index c9a3c9726..64917c737 100644
--- a/pl/threads.yap
+++ b/pl/threads.yap
@@ -337,7 +337,8 @@ thread_detach(Id) :-
 	(	recorded('$thread_exit_status', [Id0|_], _) ->
 		'$erase_thread_info'(Id0),
 		'$thread_destroy'(Id0)
-	;	true
+	;
+		'$thread_unlock'(Id0)
 	).
 
 thread_exit(Term) :-
@@ -637,6 +638,7 @@ message_queue_destroy(Queue) :-
 	recorded('$queue',q(Queue,Mutex,Cond,_,QKey),R), !,
 	erase(R),
 	'$cond_destroy'(Cond),
+	write(Queue:Mutex),nl,
 	'$destroy_mutex'(Mutex),
 	'$unlock_mutex'(QMutex),
 	'$clean_mqueue'(QKey).