diff --git a/core/modules/simpletest/src/Tests/SimpleTestBrowserTest.php b/core/modules/simpletest/src/Tests/SimpleTestBrowserTest.php
index c76e315..478aff3 100644
--- a/core/modules/simpletest/src/Tests/SimpleTestBrowserTest.php
+++ b/core/modules/simpletest/src/Tests/SimpleTestBrowserTest.php
@@ -110,6 +110,7 @@ public function testUserAgentValidation() {
     $this->assertResponse(200, 'Requesting https.php with a legitimate simpletest User-Agent returns OK.');
 
     // Now slightly modify the HMAC on the header, which should not validate.
+    $test_ua = substr_replace($test_ua, 'X;', strrpos($test_ua, ';'), 1);
     $this->additionalCurlOptions = array(CURLOPT_USERAGENT => $test_ua . 'X');
     $this->drupalGet($HTTP_path);
     $this->assertResponse(403, 'Requesting http.php with a bad simpletest User-Agent fails.');