diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index 13f9dd6..de7e2aa 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -309,9 +309,10 @@ function simpletest_phpunit_run_command(array $unescaped_test_classnames, $phpun
   // functional tests can connect to the database.
   putenv('SIMPLETEST_DB=' . Database::getConnectionInfoAsUrl());
 
-  // Setup an environment variable containing the base URL so that functional
-  // tests can browser the site under test.
-  $existing_simpletest_base_url = getenv('SIMPLETEST_BASE_URL');
+  // Setup an environment variable containing the base URL, if it is available.
+  // This allows functional tests to browse the site under test. When running
+  // tests via CLI, core/phpunit.xml.dist or core/scripts/run-tests.sh can set
+  // this variable.
   if ($base_url) {
     putenv('SIMPLETEST_BASE_URL=' . $base_url);
   }
@@ -353,7 +354,7 @@ function simpletest_phpunit_run_command(array $unescaped_test_classnames, $phpun
   chdir($old_cwd);
   putenv('SIMPLETEST_DB=');
   if ($base_url) {
-    putenv('SIMPLETEST_BASE_URL=' . $existing_simpletest_base_url);
+    putenv('SIMPLETEST_BASE_URL=');
   }
   return $ret;
 }