diff --git a/lasso/errors.c b/lasso/errors.c
index ec64168e..671fc1c2 100644
--- a/lasso/errors.c
+++ b/lasso/errors.c
@@ -109,6 +109,8 @@ lasso_strerror(int error_code)
 			return "Missing SAML artifact";
 		case LASSO_DS_ERROR_INVALID_SIGALG:
 			return "Invalid signature algorithm.";
+		case LASSO_REGISTRY_ERROR_KEY_EXISTS:
+			return "Key alreadys exists in the registry";
 		case LASSO_LOGOUT_ERROR_UNSUPPORTED_PROFILE:
 			return "Unsupported protocol profile";
 		case LASSO_LOGIN_ERROR_NO_DEFAULT_ENDPOINT:
diff --git a/lasso/errors.h b/lasso/errors.h
index b725c4f0..b357b937 100644
--- a/lasso/errors.h
+++ b/lasso/errors.h
@@ -167,3 +167,5 @@
 #define LASSO_DST_ERROR_MODIFY_PARTIALLY_FAILED   -1304 /* Modify partially failed : some items were correctly processed */
 #define LASSO_DST_ERROR_NEW_DATA_MISSING          -1305 /* Missing new data */
 
+/* Lasso registry */
+#define LASSO_REGISTRY_ERROR_KEY_EXISTS                 -1400 /* Key alreadys exists in the registry */
diff --git a/lasso/registry.c b/lasso/registry.c
index 8d7e6489..7fdc3bde 100644
--- a/lasso/registry.c
+++ b/lasso/registry.c
@@ -214,9 +214,12 @@ const char* lasso_registry_get_mapping(LassoRegistry *registry, const char *from
 	const char *ret = NULL;
 
 	from_ns_quark = g_quark_try_string(from_namespace);
-	to_ns_quark = g_quark_try_string(from_name);
+	to_ns_quark = g_quark_try_string(to_namespace);
+
+	if (from_ns_quark == 0 || to_ns_quark == 0) {
+		return NULL;
+	}
 
-	g_return_val_if_fail(from_ns_quark != 0 && to_ns_quark != 0, NULL);
 	ret = lasso_registry_get_functional_mapping(registry->functional_mapping, from_ns_quark, from_name, to_ns_quark);
 	if (ret == NULL) {
 		ret = lasso_registry_get_direct_mapping(registry->direct_mapping, from_ns_quark, from_name, to_ns_quark);
@@ -230,7 +233,8 @@ const char* lasso_registry_get_mapping(LassoRegistry *registry, const char *from
  *
  * Add a new mapping from a QName to a QName.
  *
- * Return value: 0 if successfull, -1 if it already exists, -2 if arguments
+ * Return value: 0 if successfull, LASSO_REGISTRY_ERROR_KEY_EXISTS if it already exists,
+ * LASSO_PARAM_ERROR_INVALID_VALUE if arguments
  * are invalid.
  */
 gint lasso_registry_add_direct_mapping(LassoRegistry *registry, const char *from_namespace,
@@ -238,10 +242,10 @@ gint lasso_registry_add_direct_mapping(LassoRegistry *registry, const char *from
 {
 	LassoRegistryDirectMappingRecord *a_record;
 
-	g_return_val_if_fail(registry && from_namespace && from_name && to_namespace && to_name, -2);
+	g_return_val_if_fail(registry && from_namespace && from_name && to_namespace && to_name, LASSO_PARAM_ERROR_INVALID_VALUE);
 
 	if (lasso_registry_get_mapping(registry, from_namespace, from_name, to_namespace)) {
-		return -1;
+		return LASSO_REGISTRY_ERROR_KEY_EXISTS;
 	}
 	a_record = g_new0(LassoRegistryDirectMappingRecord, 1);
 	a_record->from_namespace = g_quark_from_string(from_namespace);
@@ -263,7 +267,8 @@ gint lasso_registry_add_direct_mapping(LassoRegistry *registry, const char *from
  * translation_function. This functions is not forced to return a value for
  * any string, it can return NULL.
  *
- * Return value: 0 if successfull, -1 otherwise.
+ * Return value: 0 if successfull, LASSO_REGISTRY_ERROR_KEY_EXISTS if this mapping is already registered,
+ * LASSO_PARAM_ERROR_INVALID_VALUE if one the argument is invalid.
  */
 gint lasso_registry_add_functional_mapping(LassoRegistry *registry, const char *from_namespace,
 		const char *to_namespace, LassoRegistryTranslationFunction translation_function)
@@ -271,11 +276,11 @@ gint lasso_registry_add_functional_mapping(LassoRegistry *registry, const char *
 	LassoRegistryFunctionalMappingRecord *a_record;
 	GQuark to_ns_quark, from_ns_quark;
 
-	g_return_val_if_fail(registry != NULL && from_namespace != NULL && to_namespace != NULL, -2);
+	g_return_val_if_fail(registry != NULL && from_namespace != NULL && to_namespace != NULL, LASSO_PARAM_ERROR_INVALID_VALUE);
 	from_ns_quark = g_quark_from_string(from_namespace);
 	to_ns_quark = g_quark_from_string(to_namespace);
 	if (lasso_registry_get_translation_function(registry->functional_mapping, from_ns_quark, to_ns_quark)) {
-		return -1;
+		return LASSO_REGISTRY_ERROR_KEY_EXISTS;
 	}
 	a_record = g_new0(LassoRegistryFunctionalMappingRecord, 1);
 	a_record->from_namespace = from_ns_quark;
@@ -296,8 +301,8 @@ gint lasso_registry_add_functional_mapping(LassoRegistry *registry, const char *
  *
  * Add a new mapping from a QName to a QName.
  *
- * Return value: 0 if successfull, -1 if it already exists, -2 if arguments
- * are invalid.
+ * Return value: 0 if successfull, LASSO_REGISTRY_ERROR_KEY_EXISTS if this mapping is already registered,
+ * LASSO_PARAM_ERROR_INVALID_VALUE if one the argument is invalid.
  */
 gint lasso_registry_default_add_direct_mapping(const char *from_namespace,
 		const char *from_name, const char *to_namespace, const char *to_name)
@@ -318,7 +323,8 @@ gint lasso_registry_default_add_direct_mapping(const char *from_namespace,
  * the default mapping. This functions is not forced to return a value for any string, it can return
  * NULL.
  *
- * Return value: 0 if successfull, -1 otherwise.
+ * Return value: 0 if successfull, LASSO_REGISTRY_ERROR_KEY_EXISTS if this mapping is already registered,
+ * LASSO_PARAM_ERROR_INVALID_VALUE if one the argument is invalid.
  */
 gint lasso_registry_default_add_functional_mapping(const char *from_namespace,
 		const char *to_namespace, LassoRegistryTranslationFunction translation_function)
diff --git a/tests/basic_tests.c b/tests/basic_tests.c
index 83441dec..00880bd3 100644
--- a/tests/basic_tests.c
+++ b/tests/basic_tests.c
@@ -83,13 +83,15 @@ START_TEST(test06_registry_direct_mapping)
 	const char *name;
 	gint r;
 
-	r = lasso_registry_default_add_direct_mapping(LASSO_LIB_HREF,
-				"test", LASSO_LASSO_HREF,
-				"LassoTestClass");
+	r = lasso_registry_default_add_direct_mapping(LASSO_LIB_HREF, "test", LASSO_LASSO_HREF,
+			"LassoTestClass");
 	fail_unless(r == 0, "lasso_registry_default_add_direct_mapping should return 0 for new mappings");
 	name = lasso_registry_default_get_mapping(LASSO_LIB_HREF, "test", LASSO_LASSO_HREF);
 	fail_unless(name != NULL, "lasso_registry_default_get_mapping should return the recent mapping");
 	fail_unless(strcmp(name, "LassoTestClass") == 0, "lasso_registry_default_get_mapping should return LassoTestClass");
+	r = lasso_registry_default_add_direct_mapping(LASSO_LIB_HREF, "test", LASSO_LASSO_HREF,
+			"LassoTestClass");
+	fail_unless(r == LASSO_REGISTRY_ERROR_KEY_EXISTS, "lasso_registry_default_add_direct_mapping should return LASSO_REGISTRY_KEY_EXISTS when done two times");
 }
 END_TEST
 
@@ -117,6 +119,8 @@ START_TEST(test07_registry_functional_mapping)
 	name = lasso_registry_default_get_mapping(LASSO_LIB_HREF, "Assertion", LASSO_LASSO_HREF);
 	fail_unless(name != NULL, "lasso_registry_default_get_mapping should return the recent mapping");
 	fail_unless(strcmp(name, "LassoAssertion") == 0, "lasso_registry_default_get_mapping should return LassoAssertion");
+	r = lasso_registry_default_add_functional_mapping(LASSO_LIB_HREF, LASSO_LASSO_HREF, trad);
+	fail_unless(r == LASSO_REGISTRY_ERROR_KEY_EXISTS, "lasso_registry_default_add_functional_mapping should return LASSO_REGISTRY_KEY_EXISTS when done two times");
 }
 END_TEST