diff --git a/.travis.yml b/.travis.yml
index 84acabe..578e324 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,7 +2,6 @@ before_install:
   - git submodule update --init --recursive
 language: php
 php:
-  - 7.1
-  - 7.2
-  - 7.3
+  - 7.x
+  - 8.x
 script: phpunit tests/AllTests.php
diff --git a/lib/model/Tweet.php b/lib/model/Tweet.php
index 7f48c19..42ab626 100644
--- a/lib/model/Tweet.php
+++ b/lib/model/Tweet.php
@@ -35,7 +35,7 @@ public function __construct(array $array) {
 		$this->user = new User($array);
 	}
 
-	function jsonSerialize() {
+	function jsonSerialize(): array {
 		return [
 			'id'                    => $this->id,
 			'createdAt'             => $this->createdAt,
@@ -165,4 +165,4 @@ public function getUser() {
 	public function setUser(User $user) {
 		$this->user = $user;
 	}
-}
\ No newline at end of file
+}
diff --git a/lib/model/User.php b/lib/model/User.php
index 3165654..d5c81bc 100644
--- a/lib/model/User.php
+++ b/lib/model/User.php
@@ -15,7 +15,7 @@ public function __construct(array $array) {
 		$this->screenName = $array['screen_name'];
 	}
 
-	public function jsonSerialize() {
+	public function jsonSerialize(): array {
 		return [
 			'id'                => $this->id,
 			'name'              => $this->name,
@@ -56,4 +56,4 @@ public function setScreenName($screenName) {
 		$this->screenName = $screenName;
 	}
 
-}
\ No newline at end of file
+}