diff --git a/core/modules/composer_dependency/composer_dependency.install b/core/modules/composer_dependency/composer_dependency.install
index c4b7e39..ec140ff 100644
--- a/core/modules/composer_dependency/composer_dependency.install
+++ b/core/modules/composer_dependency/composer_dependency.install
@@ -32,6 +32,8 @@ function composer_dependency_requirements($phase) {
     }
   }
   if ($phase == 'runtime') {
+    // We have to check if the service throws an exception because the vendor/
+    // directory could have changed since the last run.
     $drupal_dependencies = NULL;
     try {
       $drupal_dependencies = \Drupal::service('composer_dependency.drupal_respository');
@@ -46,22 +48,12 @@ function composer_dependency_requirements($phase) {
     }
     if ($drupal_dependencies) {
       // Composer is installed as a dependency.
-      // Discover all composer.json files.
-      $composer_paths = [];
-      $module_handler = \Drupal::moduleHandler();
-      $module_list = $module_handler->getModuleList();
-      foreach ($module_list as $name => $module) {
-        $path = $module->getPath() . '/composer.json';
-        if (file_exists($path)) {
-          $composer_paths[$name] = $path;
-        }
-      }
+      $composer_paths = $drupal_dependencies->modulesWithComposerDependencies();
       // Determine if our requirements are met.
       $unmet_dependencies = [];
-      foreach ($composer_paths as $name => $json_path) {
-        $json_file = new JsonFile($json_path);
-        if (!$drupal_dependencies->areDependenciesMet($json_file)) {
-          $unmet_dependencies[] = $name;
+      foreach (array_keys($composer_paths) as $module_name) {
+        if (!$drupal_dependencies->checkForModule($module_name)) {
+          $unmet_dependencies[] = $module_name;
         }
       }
       if (count($unmet_dependencies)) {
diff --git a/core/modules/composer_dependency/composer_dependency.services.yml b/core/modules/composer_dependency/composer_dependency.services.yml
index 64cbf57..5ab69a6 100644
--- a/core/modules/composer_dependency/composer_dependency.services.yml
+++ b/core/modules/composer_dependency/composer_dependency.services.yml
@@ -6,4 +6,4 @@ services:
           - { name: parameter_service }
     composer_dependency.drupal_respository.factory:
         class: Drupal\composer_dependency\DrupalRepositoryFactory
-        arguments: ['@app.root']
+        arguments: ['@app.root', '@module_handler']
diff --git a/core/modules/composer_dependency/src/DrupalRepository.php b/core/modules/composer_dependency/src/DrupalRepository.php
index 49fe707..66c07bf 100644
--- a/core/modules/composer_dependency/src/DrupalRepository.php
+++ b/core/modules/composer_dependency/src/DrupalRepository.php
@@ -17,16 +17,69 @@
  */
 class DrupalRepository extends InstalledFilesystemRepository {
 
+  protected $moduleHandler;
+
   /**
+   * Constructor.
    *
    * @param string|SplString $app_root
+   *   Injected app root service parameter.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface|null $module_handler
+   *   Injected module handler service. We need to allow a NULL module handler
+   *   because this class can be used without services available.
    */
-  public function __construct($app_root) {
+  public function __construct($app_root, $module_handler = NULL) {
+    $this->moduleHandler = $module_handler;
     $installed_json = (string) $app_root . '/core/vendor/composer/installed.json';
     parent::__construct(new JsonFile($installed_json));
   }
 
   /**
+   * Gather a list of modules with Composer-based dependencies.
+   *
+   * @return array
+   *   A keyed array, with the name of the module as the key, and the path to
+   *   composer.json as the value.
+   *
+   * @throws \RuntimeException
+   */
+  public function modulesWithComposerDependencies() {
+    if ($this->moduleHandler) {
+      $composer_paths = [];
+      $module_list = $this->moduleHandler->getModuleList();
+      foreach ($module_list as $name => $module) {
+        $path = $module->getPath() . '/composer.json';
+        if (file_exists($path)) {
+          $composer_paths[$name] = $path;
+        }
+      }
+      return $composer_paths;
+    }
+    throw new \RuntimeException('The module handler service is unavailable to the Composer repository service.');
+  }
+
+  /**
+   * Check whether a module's Composer-based dependencies are met.
+   *
+   * @param string $module_name
+   *   The name of the module we're checking.
+   *
+   * @return bool
+   */
+  public function checkForModule($module_name) {
+    if ($this->moduleHandler) {
+      $module = $this->moduleHandler->getModule($module_name);
+      $module_composer = $module->getPath() . '/composer.json';
+      if (file_exists($module_composer)) {
+        return $this->areDependenciesMet(new JsonFile($module_composer));
+      }
+      // If there's no composer.json file, then dependencies are met.
+      return TRUE;
+    }
+    throw new \RuntimeException('The module handler service is unavailable to the Composer repository service.');
+  }
+
+  /**
    * Determine whether a composer.json file depends on a package not present.
    *
    * Excludes require-dev (for now).
diff --git a/core/modules/composer_dependency/src/DrupalRepositoryFactory.php b/core/modules/composer_dependency/src/DrupalRepositoryFactory.php
index e4b6d374..e3203ce 100644
--- a/core/modules/composer_dependency/src/DrupalRepositoryFactory.php
+++ b/core/modules/composer_dependency/src/DrupalRepositoryFactory.php
@@ -12,9 +12,11 @@
 class DrupalRepositoryFactory {
 
   protected $appRoot;
+  protected $moduleHandler;
 
-  public function __construct($app_root) {
+  public function __construct($app_root, $module_handler) {
     $this->appRoot = (string) $app_root;
+    $this->moduleHandler = $module_handler;
   }
 
   public function getDrupalRepository() {
@@ -29,7 +31,7 @@ public function getDrupalRepository() {
     // InstalledFilesystemRepository is available.
     // @todo: Determine a better way to do this.
     require_once __DIR__ . '/DrupalRepository.php';
-    return new DrupalRepository($this->appRoot);
+    return new DrupalRepository($this->appRoot, $this->moduleHandler);
   }
 
 }