diff -u b/core/lib/Drupal/Core/Config/ConfigInstaller.php b/core/lib/Drupal/Core/Config/ConfigInstaller.php
--- b/core/lib/Drupal/Core/Config/ConfigInstaller.php
+++ b/core/lib/Drupal/Core/Config/ConfigInstaller.php
@@ -157,7 +157,7 @@
    * {@inheritdoc}
    */
   public function installOptionalConfig(StorageInterface $storage = NULL, $dependency = []) {
-    $profile = $this->drupalGetPath('module', $this->drupalGetProfile());
+    $profile = $this->drupalGetProfile();
     if (!$storage) {
       // Search the install profile's optional configuration too.
       $storage = new ExtensionInstallStorage($this->getActiveStorages(StorageInterface::DEFAULT_COLLECTION), InstallStorage::CONFIG_OPTIONAL_DIRECTORY, StorageInterface::DEFAULT_COLLECTION, TRUE);
@@ -166,9 +166,14 @@
     }
     elseif (isset($profile)) {
       // Creates a profile storage to search for overrides.
-      $profile_install_path = $profile . '/' . InstallStorage::CONFIG_OPTIONAL_DIRECTORY;
+      $profile_install_path = $this->drupalGetPath('module', $profile . '/' . InstallStorage::CONFIG_OPTIONAL_DIRECTORY;
       $profile_storage = new FileStorage($profile_install_path, StorageInterface::DEFAULT_COLLECTION);
     }
+    else {
+      // Profile has not been set yet. For example during the first steps of the
+      // installer or during unit tests.
+      $profile_storage = NULL;
+    }
 
     $collection_info = $this->configManager->getConfigCollectionInfo();
     $enabled_extensions = $this->getEnabledExtensions();