diff -u b/core/modules/page_cache/tests/src/Functional/PageCacheTest.php b/core/modules/page_cache/tests/src/Functional/PageCacheTest.php --- b/core/modules/page_cache/tests/src/Functional/PageCacheTest.php +++ b/core/modules/page_cache/tests/src/Functional/PageCacheTest.php @@ -220,7 +220,10 @@ ]); $this->assertSession()->statusCodeEquals(304); - $this->drupalGet('', [], ['If-Modified-Since' => $last_modified]); + $this->drupalGet('', [], [ + 'If-Modified-Since' => $last_modified, + 'If-None-Match' => NULL, + ]); // Verify the page is not printed twice when the cache is warm. $this->assertSession()->responseNotMatches('#assertSession()->statusCodeEquals(200);