diff --git a/C/cdmgr.c b/C/cdmgr.c
index e64533b0a..6bb744401 100644
--- a/C/cdmgr.c
+++ b/C/cdmgr.c
@@ -1014,20 +1014,6 @@ bool Yap_unknown(Term t) {
   return false;
 }
 
-static Int
-undef_handler(USES_REGS1) { /* '$undef_handler'(+S,+Mod)	 */
-  PredEntry *pe;
-  Int out;
-
-  pe = get_pred(Deref(ARG1), Deref(ARG2), "undef_handler");
-  if (EndOfPAEntr(pe))
-    return FALSE;
-  PELOCK(27, pe);
-  UndefCode = pe;
-  UNLOCKPE(44, pe);
-  return true;
-}
-
 static int source_pred(PredEntry *p, yamop *q) {
   if (p->PredFlags & (DynamicPredFlag | LogUpdatePredFlag))
     return FALSE;
@@ -2701,16 +2687,16 @@ static Int undefp_handler(USES_REGS1) { /* '$undefp_handler'(P,Mod)	 */
   PredEntry *pe;
 
   pe = get_pred(Deref(ARG1), Deref(ARG2), "undefined/1");
-  PELOCK(59, pe);
   if (EndOfPAEntr(pe))
     return false;
+  PELOCK(59, pe);
   if (pe->OpcodeOfPred == UNDEF_OPCODE) {
-    UNLOCKPE(58, pe);
+    UNLOCKPE(59, pe);
     return false;
   }
   UndefCode = pe;
   UNLOCKPE(59, pe);
-  return FALSE;
+  return true;
 }
 
 static Int p_undefined(USES_REGS1) { /* '$undefined'(P,Mod)	 */
@@ -4667,7 +4653,6 @@ void Yap_InitCdMgr(void) {
   Yap_InitCPred("$call_count_reset", 0, p_call_count_reset,
                 SafePredFlag | SyncPredFlag);
   Yap_InitCPred("$set_pred_module", 2, p_set_pred_module, SafePredFlag);
-  Yap_InitCPred("$undef_handler", 2, undef_handler, SafePredFlag);
   Yap_InitCPred("$set_pred_owner", 2, p_set_pred_owner, SafePredFlag);
   Yap_InitCPred("$hide_predicate", 2, hide_predicate, SafePredFlag);
   Yap_InitCPred("$stash_predicate", 2, p_stash_predicate, SafePredFlag);
diff --git a/pl/boot.yap b/pl/boot.yap
index 054e7d838..344355083 100644
--- a/pl/boot.yap
+++ b/pl/boot.yap
@@ -266,27 +266,27 @@ private(_).
     !,
     format(user_error, '~a:~d:0: unprocessed ~a ~w ~n', [F0, L,Level,Msg]).
 '$early_print_message'(Level, Msg) :-
-	format(user_error, 'unprocessed ~a ~w ~n', [Level,Msg]).
+    format(user_error, 'unprocessed ~a ~w ~n', [Level,Msg]).
 
- '$handle_error'(_Action,_G0,_M0) :- fail.
+'$handle_error'(_Action,_G0,_M0) :- fail.
 
 % cases where we cannot afford to ever fail.
 '$undefp0'([ImportingMod|G], _) :-
-	recorded('$import','$import'(ExportingModI,ImportingMod,G,G0I,_,_),_), !,
- % writeln('$execute0'(G0I, ExportingModI)),
-	'$execute0'(G0I, ExportingModI).
-'$undefp0'([_|print_message(Context, Msg)], _) :- !,
+    recorded('$import','$import'(ExportingModI,ImportingMod,G,G0I,_,_),_), !,
+    % writeln('$execute0'(G0I, ExportingModI)),
+    '$execute0'(G0I, ExportingModI).
+'$undefp0'([_|print_message(Context, Msg)], _) :-
+    !,
     '$early_print_message'(Context, Msg).
 % undef handler
 '$undefp0'([M0|G0], Action) :-
     % make sure we do not loop on undefined predicates
-    yap_flag( unknown, Action, fail),
-    Action \= fail,
-     '$handle_error'(Action,G0,M0),
-     clause_location(Call, Caller),
-     source_module(M),
-	 strip_module(M:Goal,M1,NGoal),
-	throw(error(Error, [[g|g(M1:NGoal)],[p|Call],[e|Caller],[h|g(Head)]])).
+    yap_flag( unknown, Action, fail),	    
+    '$handle_error'(Action,G0,M0),
+    clause_location(Call, Caller),
+    source_module(M),
+    strip_module(M0:G0,M1,NGoal),
+    throw(error(evaluation(undefined,M0:G0), [[g|g(M1:NGoal)],[p|Call],[e|Caller],[h|g(M0:G0)]])).
 
 
 
diff --git a/pl/messages.yap b/pl/messages.yap
index ef7fdcd8d..85f6ee581 100644
--- a/pl/messages.yap
+++ b/pl/messages.yap
@@ -931,40 +931,6 @@ prolog:print_message(Severity, _Term) :-
 	format('No handler for ~a message ~q,~n',[Severity, _Term]).
 
 
-% cases where we cannot afford to ever fail.
-'$undefp'([ImportingMod|G], _) :-
-	recorded('$import','$import'(ExportingModI,ImportingMod,G,G0I,_,_),_), !,
- % writeln('$execute'(G0I, ExportingModI)),
-	'$execute0'(G0I, ExportingModI).
-% undef handler
-'$undefp'([M0|G0], Action) :-
-    % make sure we do not loop on undefined predicates
-    '$stop_creeping'(Current),
-    yap_flag( unknown, Action, fail),
-    Action\=fail,
- %   yap_flag( debug, Debug, false),
-    (
-     '$undefp_search'(M0:G0, NM:NG),
-     ( M0 \== NM -> true  ; G0 \== NG ),
-     NG \= fail
-	->
-     yap_flag( unknown, _, Action),
-       %   yap_flag( debug, _, Debug),
-     (
-       Current == true
-      ->
-       % carry on signal processing
-       '$start_creep'([NM|NG], creep)
-     ;
-       '$execute0'(NG, NM)
-     )
-	;
-     yap_flag( unknown, _, Action),
-     '$handle_error'(Action,G0,M0)
-    ).
-
-:- '$undef_handler'('$undefp'(_,_), prolog).
-
 /**
   @}
 */
diff --git a/pl/undefined.yap b/pl/undefined.yap
index 48bde51a6..64eb6c23e 100644
--- a/pl/undefined.yap
+++ b/pl/undefined.yap
@@ -105,6 +105,34 @@ followed by the failure of that call.
     expand_goal(MG, FMG).
 
 
+% undef handler
+'$undefp'([M0|G0], Action) :-
+    % make sure we do not loop on undefined predicates
+    yap_flag( unknown, Action, fail),
+    '$stop_creeping'(Current),
+ %   yap_flag( debug, Debug, false),
+    (
+     '$undefp_search'(M0:G0, NM:NG),
+     ( M0 \== NM -> true  ; G0 \== NG ),
+     NG \= fail
+     ->
+	 yap_flag( unknown, _, Action),
+	 %   yap_flag( debug, _, Debug),
+	 (
+	     Current == true
+	  ->
+	      % carry on signal processing
+	      '$start_creep'([NM|NG], creep)
+	  ;
+	  '$execute0'(NG, NM)
+	 )
+     ;
+     yap_flag( unknown, _, Action),
+     '$handle_error'(Action,G0,M0)
+    ).
+
+:- '$undefp_handler'('$undefp'(_,_), prolog).
+
 /** @pred  unknown(- _O_,+ _N_)
 
 The unknown predicate, informs about what the user wants to be done