diff --git a/src/libecmaobjects/ecma-helpers.c b/src/libecmaobjects/ecma-helpers.c
index 763b55bd0..690c765bc 100644
--- a/src/libecmaobjects/ecma-helpers.c
+++ b/src/libecmaobjects/ecma-helpers.c
@@ -411,6 +411,8 @@ ecma_property_t*
 ecma_create_internal_property (ecma_object_t *object_p, /**< the object */
                                ecma_internal_property_id_t property_id) /**< internal property identifier */
 {
+  JERRY_ASSERT (ecma_find_internal_property (object_p, property_id) == NULL);
+
   ecma_property_t *new_property_p = ecma_alloc_property ();
 
   new_property_p->type = ECMA_PROPERTY_INTERNAL;
@@ -489,6 +491,7 @@ ecma_create_named_data_property (ecma_object_t *obj_p, /**< object */
                                  ecma_property_configurable_value_t configurable) /**< 'configurable' attribute */
 {
   JERRY_ASSERT(obj_p != NULL && name_p != NULL);
+  JERRY_ASSERT(ecma_find_named_property (obj_p, name_p) == NULL);
 
   ecma_property_t *prop_p = ecma_alloc_property ();
 
@@ -528,6 +531,7 @@ ecma_create_named_accessor_property (ecma_object_t *obj_p, /**< object */
                                      ecma_property_configurable_value_t configurable) /**< 'configurable' attribute */
 {
   JERRY_ASSERT(obj_p != NULL && name_p != NULL);
+  JERRY_ASSERT(ecma_find_named_property (obj_p, name_p) == NULL);
 
   ecma_property_t *prop_p = ecma_alloc_property ();
 
diff --git a/src/libecmaoperations/ecma-lex-env.c b/src/libecmaoperations/ecma-lex-env.c
index 5044b9c13..91688a6d2 100644
--- a/src/libecmaoperations/ecma-lex-env.c
+++ b/src/libecmaoperations/ecma-lex-env.c
@@ -68,7 +68,7 @@ ecma_op_has_binding (ecma_object_t *lex_env_p, /**< lexical environment */
 /**
  * CreateMutableBinding operation.
  *
- * see also: ecma-262 v5, 10.2.1
+ * See also: ECMA-262 v5, 10.2.1
  *
  * @return completion value
  *         Returned value must be freed with ecma_free_completion_value
@@ -86,8 +86,6 @@ ecma_op_create_mutable_binding (ecma_object_t *lex_env_p, /**< lexical environme
   {
     case ECMA_LEXICAL_ENVIRONMENT_DECLARATIVE:
     {
-      JERRY_ASSERT(!ecma_op_has_binding (lex_env_p, name_p));
-
       ecma_create_named_data_property (lex_env_p,
                                        name_p,
                                        ECMA_PROPERTY_WRITABLE,
@@ -155,8 +153,6 @@ ecma_op_set_mutable_binding (ecma_object_t *lex_env_p, /**< lexical environment
                && ecma_is_lexical_environment (lex_env_p));
   JERRY_ASSERT(name_p != NULL);
 
-  JERRY_ASSERT(ecma_op_has_binding (lex_env_p, name_p));
-
   switch (ecma_get_lex_env_type (lex_env_p))
   {
     case ECMA_LEXICAL_ENVIRONMENT_DECLARATIVE:
@@ -236,8 +232,6 @@ ecma_op_get_binding_value (ecma_object_t *lex_env_p, /**< lexical environment */
                && ecma_is_lexical_environment (lex_env_p));
   JERRY_ASSERT(name_p != NULL);
 
-  JERRY_ASSERT(ecma_op_has_binding (lex_env_p, name_p));
-
   switch (ecma_get_lex_env_type (lex_env_p))
   {
     case ECMA_LEXICAL_ENVIRONMENT_DECLARATIVE:
@@ -422,8 +416,6 @@ ecma_op_create_immutable_binding (ecma_object_t *lex_env_p, /**< lexical environ
   {
     case ECMA_LEXICAL_ENVIRONMENT_DECLARATIVE:
     {
-      JERRY_ASSERT(!ecma_op_has_binding (lex_env_p, name_p));
-
       /*
        * Warning:
        *         Whether immutable bindings are deletable seems not to be defined by ECMA v5.
@@ -466,8 +458,6 @@ ecma_op_initialize_immutable_binding (ecma_object_t *lex_env_p, /**< lexical env
   {
     case ECMA_LEXICAL_ENVIRONMENT_DECLARATIVE:
     {
-      JERRY_ASSERT(ecma_op_has_binding (lex_env_p, name_p));
-
       ecma_property_t *prop_p = ecma_get_named_data_property (lex_env_p, name_p);
 
       /* The binding must be unitialized immutable binding */