diff --git a/library/map.m b/library/map.m
index f2de80295..1e386e9d0 100644
--- a/library/map.m
+++ b/library/map.m
@@ -225,12 +225,12 @@ map__lookup(Map, K, V) :-
 	( tree234__search(Map, K, V1) ->
 		V = V1
 	;
-		map__lookup_error(K, V)
+		map__lookup_error("map__lookup: key_not_found", K, V)
 	).
 
-:- pred map__lookup_error(K, V).
-:- mode map__lookup_error(in, unused) is erroneous.
-map__lookup_error(K, V) :-
+:- pred map__lookup_error(string, K, V).
+:- mode map__lookup_error(in, in, unused) is erroneous.
+map__lookup_error(Msg, K, V) :-
 	KeyType = type_name(type_of(K)),
 	ValueType = type_name(type_of(V)),
 	functor(K, Functor, Arity),
@@ -242,8 +242,8 @@ map__lookup_error(K, V) :-
 			FunctorStr)
 	),
 	string__append_list(
-		["map__lookup: key not found\n",
-		"\tKey Type: ",
+		[Msg,
+		"\n\tKey Type: ",
 		KeyType,
 		"\n\tKey Functor: ",
 		FunctorStr,
@@ -260,7 +260,7 @@ map__det_insert(Map0, K, V, Map) :-
 	( tree234__insert(Map0, K, V, Map1) ->
 		Map = Map1
 	;
-		error("map__det_insert: key already present")
+		map__lookup_error("map__det_insert: key already present", K, V)
 	).
 
 map__det_insert_from_corresponding_lists(Map0, Ks, Vs, Map) :-
@@ -283,7 +283,7 @@ map__det_update(Map0, K, V, Map) :-
 	( tree234__update(Map0, K, V, Map1) ->
 		Map = Map1
 	;
-		error("map__det_update: key not found")
+		map__lookup_error("map__det_update: key not found", K, V)
 	).
 
 map__set(Map0, K, V, Map) :-
@@ -320,7 +320,7 @@ map__det_remove(Map0, Key, Value, Map) :-
 		Value = Value1,
 		Map = Map1
 	;
-		error("map__det_remove: key not found")
+		map__lookup_error("map__det_remove: key not found", Key, Value)
 	).
 
 map__count(Map, Count) :-
diff --git a/library/map.nu.nl b/library/map.nu.nl
index 8f7df5417..d89de84e4 100644
--- a/library/map.nu.nl
+++ b/library/map.nu.nl
@@ -1,5 +1,5 @@
 %---------------------------------------------------------------------------%
-% Copyright (C) 1997 The University of Melbourne.
+% Copyright (C) 1997-1998 The University of Melbourne.
 % This file may only be copied under the terms of the GNU Library General
 % Public License - see the file COPYING.LIB in the Mercury distribution.
 %---------------------------------------------------------------------------%
@@ -37,7 +37,7 @@ map__det_insert(Map0, K, V, Map) :-
 % map__det_insert_error is a separate predicate because it is handy
 % to be able to set a spy point on it...
 map__det_insert_error(_Map, _K, _V) :-
-	error("map__det_insert: key not found").
+	error("map__det_insert: key already present").
 
 %-----------------------------------------------------------------------------%
 %-----------------------------------------------------------------------------%