diff --git a/Json.cc b/Json.cc
index 2505a7a..936ac62 100644
--- a/Json.cc
+++ b/Json.cc
@@ -52,7 +52,7 @@ json_value_t* json_value_copy(const json_value_t* val)
 
 // ------------------------ Constructor -------------------------
 Json::Json()
-    : node_(json_value_create(JSON_VALUE_NULL)), parent_(nullptr),
+    : node_(json_value_create(-1)), parent_(nullptr),
       allocated_(true)
 {
 }
@@ -231,7 +231,7 @@ std::string Json::dump(int spaces) const
 
 Json Json::operator[](const char* key)
 {
-    if (is_null() && is_root())
+    if (is_placeholder() && is_root())
     {
         // todo : need is_root here?
         to_object();
@@ -369,7 +369,7 @@ bool Json::can_obj_push_back()
     {
         return true;
     }
-    if (is_root() && is_null())
+    if (is_root() && is_placeholder())
     {
         to_object();
     }
@@ -552,15 +552,18 @@ void Json::normal_push_back(const std::string& key,
 {
     json_object_t* obj = json_value_object(parent_);
     const json_value_t* find = json_object_find(key.c_str(), obj);
+    const json_value_t* v;
     if (find == nullptr)
     {
-        json_object_append(obj, key.c_str(), JSON_VALUE_NULL);
-        return;
+        v = json_object_append(obj, key.c_str(), JSON_VALUE_ARRAY);
+    }
+    else
+    {
+        v = json_object_insert_before(find, obj, key.c_str(),
+                                      JSON_VALUE_ARRAY);
+        json_value_t* remove_val = json_object_remove(find, obj);
+        json_value_destroy(remove_val);
     }
-    const json_value_t *v = json_object_insert_before(find, obj, key.c_str(),
-                                                      JSON_VALUE_ARRAY);
-    json_value_t* remove_val = json_object_remove(find, obj);
-    json_value_destroy(remove_val);
     json_array_t* arr = json_value_array(v);
     for (const auto& str : val)
         json_array_append(arr, JSON_VALUE_STRING, str.c_str());
@@ -585,7 +588,7 @@ void Json::normal_push_back(const std::string& key, const Json& val)
 
 bool Json::can_arr_push_back()
 {
-    if (is_root() && is_null())
+    if (is_root() && is_placeholder())
     {
         to_array();
     }
@@ -746,16 +749,12 @@ bool Json::empty() const
 {
     switch (type())
     {
+        case -1:
         case JSON_VALUE_NULL:
-        {
-            // null values are empty
             return true;
-        }
         case JSON_VALUE_ARRAY:
         case JSON_VALUE_OBJECT:
-        {
             return size() == 0;
-        }
         default:
             // all other types are nonempty
             return false;
@@ -786,7 +785,7 @@ void Json::clear()
 
 bool Json::to_object()
 {
-    if (!allocated_ || !is_null())
+    if (!allocated_ || !is_placeholder())
     {
         // watcher and non-null type can't change type
         return false;
@@ -799,7 +798,7 @@ bool Json::to_object()
 
 bool Json::to_array()
 {
-    if (!allocated_ || !is_null())
+    if (!allocated_ || !is_placeholder())
     {
         // watcher and non-null type can't change type
         return false;
diff --git a/Json.h b/Json.h
index 14a06ae..060261b 100644
--- a/Json.h
+++ b/Json.h
@@ -175,6 +175,12 @@ class Json
         return type() == JSON_VALUE_STRING;
     }
 
+    // can convert to any type, just like a placeholder
+    bool is_placeholder() const
+    {
+        return json_value_type(node_) == -1;
+    }
+
     bool is_valid() const
     {
         return node_ != nullptr;
@@ -684,12 +690,6 @@ class Json
 
     bool can_arr_push_back();
 
-    // can convert to any type, just like a placeholder
-    bool is_placeholder() const
-    {
-        return is_null() && parent_ != nullptr;
-    }
-
     void destroy_node(json_value_t** node)
     {
         if (allocated_)
diff --git a/test/json_obj_test.cc b/test/json_obj_test.cc
index a197a36..84c0c8f 100644
--- a/test/json_obj_test.cc
+++ b/test/json_obj_test.cc
@@ -19,9 +19,9 @@ TEST(ObjTest, empty_obj)
 TEST(ObjTest, one_level)
 {
     Json data;
-    EXPECT_TRUE(data.is_null());
+    EXPECT_TRUE(data.is_placeholder());
     Json data_tmp = data["test"];
-    EXPECT_TRUE(data_tmp.is_null());
+    EXPECT_TRUE(data_tmp.is_placeholder());
     data["test"] = 1.0;
     EXPECT_EQ(data.type(), JSON_VALUE_OBJECT);
     EXPECT_TRUE(data.is_object());