diff --git a/lib/Db/Attach.php b/lib/Db/Attach.php
index 4be6f16..58f7996 100644
--- a/lib/Db/Attach.php
+++ b/lib/Db/Attach.php
@@ -38,7 +38,7 @@ public function setDeepLinkUrl(string $deepLinkUrl): void {
 		$this->deepLinkUrl = $deepLinkUrl;
 	}
 
-	public function jsonSerialize() {
+	public function jsonSerialize(): array {
 		return [
 			'id'            => $this->id,
 			'note_id'       => $this->noteId,
diff --git a/lib/Db/Color.php b/lib/Db/Color.php
index 181286c..e4ad4ae 100644
--- a/lib/Db/Color.php
+++ b/lib/Db/Color.php
@@ -13,7 +13,7 @@ class Color extends Entity implements JsonSerializable {
 
 	protected $color;
 
-	public function jsonSerialize() {
+	public function jsonSerialize(): array {
 		return [
 			'id' => $this->id,
 			'color' => $this->color
diff --git a/lib/Db/Note.php b/lib/Db/Note.php
index d962560..e55da9c 100644
--- a/lib/Db/Note.php
+++ b/lib/Db/Note.php
@@ -65,7 +65,7 @@ public function setAttachts(array $attachts): void {
 		$this->attachts = $attachts;
 	}
 
-	public function jsonSerialize() {
+	public function jsonSerialize(): array {
 		return [
 			'id'          => $this->id,
 			'title'       => $this->title,
diff --git a/lib/Db/NoteShare.php b/lib/Db/NoteShare.php
index a6fbd66..855d537 100644
--- a/lib/Db/NoteShare.php
+++ b/lib/Db/NoteShare.php
@@ -30,7 +30,7 @@ public function setDisplayName (string $displayName): void {
 		$this->displayName = $displayName;
 	}
 
-	public function jsonSerialize() {
+	public function jsonSerialize(): array {
 		return [
 			'id' => $this->id,
 			'note_id' => $this->noteId,
diff --git a/lib/Db/NoteTag.php b/lib/Db/NoteTag.php
index d7edd6a..d8242b3 100644
--- a/lib/Db/NoteTag.php
+++ b/lib/Db/NoteTag.php
@@ -19,7 +19,7 @@ class NoteTag extends Entity implements JsonSerializable {
 	protected $tagId;
 	protected $userId;
 
-	public function jsonSerialize() {
+	public function jsonSerialize(): array {
 		return [
 			'id' => $this->id,
 			'noteid' => $this->noteId,
diff --git a/lib/Db/Tag.php b/lib/Db/Tag.php
index caee739..f19807b 100644
--- a/lib/Db/Tag.php
+++ b/lib/Db/Tag.php
@@ -16,7 +16,7 @@ class Tag extends Entity implements JsonSerializable {
 	protected $userId;
 	protected $name;
 
-	public function jsonSerialize() {
+	public function jsonSerialize(): array {
 		return [
 			'id' => $this->id,
 			'userid' => $this->userId,