diff --git a/src/libecmaobjects/ecma-helpers.c b/src/libecmaobjects/ecma-helpers.c
index c906c181d..b63f87462 100644
--- a/src/libecmaobjects/ecma-helpers.c
+++ b/src/libecmaobjects/ecma-helpers.c
@@ -395,7 +395,6 @@ ecma_free_internal_property (ecma_property_t *property_p) /**< the property */
   {
     case ECMA_INTERNAL_PROPERTY_NUMBER_INDEXED_ARRAY_VALUES: /* a collection */
     case ECMA_INTERNAL_PROPERTY_STRING_INDEXED_ARRAY_VALUES: /* a collection */
-    case ECMA_INTERNAL_PROPERTY_FORMAL_PARAMETERS: /* a collection */
     {
       TODO (/* Free collection's elements */);
       JERRY_UNIMPLEMENTED();
@@ -404,6 +403,15 @@ ecma_free_internal_property (ecma_property_t *property_p) /**< the property */
       break;
     }
 
+    case ECMA_INTERNAL_PROPERTY_FORMAL_PARAMETERS: /* a strings' collection */
+    {
+      if (property_value != ECMA_NULL_POINTER)
+      {
+        ecma_free_strings_collection (ECMA_GET_POINTER(property_value));
+      }
+      break;
+    }
+
     case ECMA_INTERNAL_PROPERTY_SCOPE: /* a lexical environment */
     case ECMA_INTERNAL_PROPERTY_BINDING_OBJECT: /* an object */
     case ECMA_INTERNAL_PROPERTY_PROTOTYPE: /* the property's value is located in ecma_object_t */
diff --git a/src/libecmaoperations/ecma-function-object.c b/src/libecmaoperations/ecma-function-object.c
index dcaee5ee3..e8f2a0d57 100644
--- a/src/libecmaoperations/ecma-function-object.c
+++ b/src/libecmaoperations/ecma-function-object.c
@@ -108,8 +108,8 @@ ecma_op_is_callable (ecma_value_t value) /**< ecma-value */
  * @return pointer to newly created Function object
  */
 ecma_object_t*
-ecma_op_create_function_object (const ecma_char_t* formal_parameter_list_p[], /**< formal parameters list */
-                                uint32_t formal_parameters_number, /**< formal parameters list's length */
+ecma_op_create_function_object (ecma_string_t* formal_parameter_list_p[], /**< formal parameters list */
+                                ecma_length_t formal_parameters_number, /**< formal parameters list's length */
                                 ecma_object_t *scope_p, /**< function's scope */
                                 bool is_strict, /**< 'strict' flag */
                                 opcode_counter_t first_opcode_idx) /**< index of first opcode of function's body */
@@ -137,9 +137,17 @@ ecma_op_create_function_object (const ecma_char_t* formal_parameter_list_p[], /*
   ecma_gc_update_may_ref_younger_object_flag_by_object (f, scope_p);
 
   // 10., 11.
+  ecma_property_t *formal_parameters_prop_p = ecma_create_internal_property (f,
+                                                                             ECMA_INTERNAL_PROPERTY_FORMAL_PARAMETERS);
   if (formal_parameters_number != 0)
   {
-    JERRY_UNIMPLEMENTED_REF_UNUSED_VARS(formal_parameter_list_p);
+    ecma_collection_header_t *formal_parameters_collection_p = ecma_new_strings_collection (formal_parameter_list_p,
+                                                                                            formal_parameters_number);
+    ECMA_SET_POINTER (formal_parameters_prop_p->u.internal_property.value, formal_parameters_collection_p);
+  }
+  else
+  {
+    JERRY_ASSERT (formal_parameters_prop_p->u.internal_property.value == ECMA_NULL_POINTER);
   }
 
   // 12.
diff --git a/src/libecmaoperations/ecma-function-object.h b/src/libecmaoperations/ecma-function-object.h
index 01bfec482..63c9d238d 100644
--- a/src/libecmaoperations/ecma-function-object.h
+++ b/src/libecmaoperations/ecma-function-object.h
@@ -29,8 +29,8 @@
 extern bool ecma_op_is_callable (ecma_value_t value);
 
 extern ecma_object_t*
-ecma_op_create_function_object (const ecma_char_t* formal_parameter_list_p[],
-                                uint32_t formal_parameters_number,
+ecma_op_create_function_object (ecma_string_t* formal_parameter_list_p[],
+                                ecma_length_t formal_parameters_number,
                                 ecma_object_t *scope_p,
                                 bool is_strict,
                                 opcode_counter_t first_opcode_idx);