diff --git a/tests/phpunit/includes/testcase-rest-controller.php b/tests/phpunit/includes/testcase-rest-controller.php
index a0b96545bf..67a7970b66 100644
--- a/tests/phpunit/includes/testcase-rest-controller.php
+++ b/tests/phpunit/includes/testcase-rest-controller.php
@@ -45,7 +45,14 @@ abstract class WP_Test_REST_Controller_Testcase extends WP_Test_REST_TestCase {
 		}
 
 		// Make sure path for rest_url has a leading slash for proper resolution.
-		$this->assertTrue( 0 === strpos( $path, '/' ), 'REST API URL should have a leading slash.' );
+		if ( 0 !== strpos( $path, '/' ) ) {
+			$this->fail(
+				sprintf(
+					'REST API URL "%s" should have a leading slash.',
+					$path
+				)
+			);
+		}
 
 		return $url;
 	}