diff --git a/todoist/id.go b/todoist/id.go
index ae33e01..d7080f9 100644
--- a/todoist/id.go
+++ b/todoist/id.go
@@ -34,6 +34,9 @@ func (i ID) MarshalJSON() ([]byte, error) {
 	if IsTempID(i) {
 		s = `"` + s + `"`
 	}
+	if s == "0" {
+		s = "null"
+	}
 	return []byte(s), nil
 }
 
@@ -42,6 +45,9 @@ func (i *ID) UnmarshalJSON(b []byte) (err error) {
 	if err != nil {
 		s = string(b) // integer id
 	}
+	if s == "null" {
+		s = "0"
+	}
 	id, err := NewID(s)
 	if err != nil {
 		return err
diff --git a/todoist/id_test.go b/todoist/id_test.go
index 02fed1e..3341a1e 100644
--- a/todoist/id_test.go
+++ b/todoist/id_test.go
@@ -52,6 +52,11 @@ func TestID_MarshalJSON(t *testing.T) {
 	if err != nil || string(b) != strconv.Quote(test.s) {
 		t.Errorf("Expect %s, but got %s", strconv.Quote(test.s), string(b))
 	}
+
+	b, err = ID("0").MarshalJSON()
+	if err != nil || string(b) != "null" {
+		t.Errorf("Expect %s, but got %s", strconv.Quote(test.s), string(b))
+	}
 }
 
 func TestID_UnmarshalJSON(t *testing.T) {
@@ -61,9 +66,17 @@ func TestID_UnmarshalJSON(t *testing.T) {
 		if !reflect.DeepEqual(err, test.e) {
 			t.Errorf("Expect %s, but got %s", test.e, err)
 		} else if test.e == nil && v != test.v {
-
+			t.Errorf("Expect %s, but got %s", test.v, v)
 		}
 	}
+	var v ID
+	err := v.UnmarshalJSON([]byte("null"))
+	if err != nil {
+		t.Errorf("Unexpect error: %s", err)
+	}
+	if v != ID("0") {
+		t.Errorf("Expect %s, but got %s", ID("0"), v)
+	}
 }
 
 func TestIsTempID(t *testing.T) {