diff --git a/tests/Unit/Math/PrimeFieldTest.php b/tests/Unit/Math/PrimeFieldTest.php
index 24ecfabd..34b1dd81 100644
--- a/tests/Unit/Math/PrimeFieldTest.php
+++ b/tests/Unit/Math/PrimeFieldTest.php
@@ -8,7 +8,7 @@ use phpseclib3\Tests\PhpseclibTestCase;
 
 class PrimeFieldTest extends PhpseclibTestCase
 {
-    public function testPrimeFieldWithCompositeNumbers()
+    public function testPrimeFieldWithCompositeNumbers(): void
     {
         $this->expectException('UnexpectedValueException');