diff --git a/tests/phpunit/TemplateStylesContentTest.php b/tests/phpunit/TemplateStylesContentTest.php
index 847eab3..f195f46 100644
--- a/tests/phpunit/TemplateStylesContentTest.php
+++ b/tests/phpunit/TemplateStylesContentTest.php
@@ -157,7 +157,7 @@ class TemplateStylesContentTest extends TextContentTest {
 	/**
 	 * @dataProvider dataEquals
 	 */
-	public function testEquals( Content $a, Content $b = null, $equal = false ) {
+	public function testEquals( Content $a, ?Content $b = null, $equal = false ) {
 		$this->assertEquals( $equal, $a->equals( $b ) );
 	}
 }