diff --git a/tests/Unit/Math/PrimeFieldTest.php b/tests/Unit/Math/PrimeFieldTest.php
index 5d003b01..3f6f10f1 100644
--- a/tests/Unit/Math/PrimeFieldTest.php
+++ b/tests/Unit/Math/PrimeFieldTest.php
@@ -35,6 +35,6 @@ class PrimeFieldTest extends PhpseclibTestCase
             new \phpseclib3\Crypt\EC\Curves\secp192r1()
         );
 
-        $point[0]->toBytes();
+        $this->assertIsString($point[0]->toBytes());
     }
 }