diff --git a/core/tests/Drupal/Tests/ComposerIntegrationTest.php b/core/tests/Drupal/Tests/ComposerIntegrationTest.php
index 85ac83b..8ee025a 100644
--- a/core/tests/Drupal/Tests/ComposerIntegrationTest.php
+++ b/core/tests/Drupal/Tests/ComposerIntegrationTest.php
@@ -198,6 +198,11 @@ public function testMinPHPVersion() {
 
     $lock = json_decode(file_get_contents($lockfile), TRUE);
 
+    // Check the PHP version for each installed non-development  package. The
+    // testing infrastructure uses the uses the development packages, and may
+    // update them for particular environment configurations. In particular,
+    // PHP 7.2+ require an updated version of phpunit, which is incompatible
+    // with Drupal's minimum PHP requirement.
     foreach ($lock['packages'] as $package) {
       if (isset($package['require']['php'])) {
         $this->assertTrue(Semver::satisfies(static::MIN_PHP_VERSION, $package['require']['php']), $package['name'] . ' has a PHP dependency requirement of "' . $package['require']['php'] . '"');