diff --git a/tests/CSSList/DocumentTest.php b/tests/CSSList/DocumentTest.php
index a727400b..8c0fffdf 100644
--- a/tests/CSSList/DocumentTest.php
+++ b/tests/CSSList/DocumentTest.php
@@ -18,7 +18,7 @@ class DocumentTest extends TestCase
      */
     private $subject;
 
-    protected function setUp()
+    protected function setUp(): void
     {
         $this->subject = new Document();
     }
diff --git a/tests/CSSList/KeyFrameTest.php b/tests/CSSList/KeyFrameTest.php
index 080d5f94..b8620eb4 100644
--- a/tests/CSSList/KeyFrameTest.php
+++ b/tests/CSSList/KeyFrameTest.php
@@ -18,7 +18,7 @@ class KeyFrameTest extends TestCase
      */
     protected $subject;
 
-    protected function setUp()
+    protected function setUp(): void
     {
         $this->subject = new KeyFrame();
     }
diff --git a/tests/OutputFormatTest.php b/tests/OutputFormatTest.php
index 0de39123..02d4ee93 100644
--- a/tests/OutputFormatTest.php
+++ b/tests/OutputFormatTest.php
@@ -43,7 +43,7 @@ class OutputFormatTest extends TestCase
      */
     private $oDocument;
 
-    protected function setUp()
+    protected function setUp(): void
     {
         $this->oParser = new Parser(self::TEST_CSS);
         $this->oDocument = $this->oParser->parse();