diff --git a/src/DependencyInjection/LeagueOAuth2ServerExtension.php b/src/DependencyInjection/LeagueOAuth2ServerExtension.php
index 81922ad0..d7f30bca 100644
--- a/src/DependencyInjection/LeagueOAuth2ServerExtension.php
+++ b/src/DependencyInjection/LeagueOAuth2ServerExtension.php
@@ -81,6 +81,13 @@ public function getAlias(): string
      */
     public function prepend(ContainerBuilder $container)
     {
+        // If no doctrine connection is configured, the DBAL connection should
+        // be left alone as adding any configuration setting with no connection
+        // will result in an invalid configuration leading to a hard failure.
+        if (!self::hasDoctrineConnectionsConfigured($container->getExtensionConfig('doctrine'))) {
+            return;
+        }
+
         $container->prependExtensionConfig('doctrine', [
             'dbal' => [
                 'connections' => null,
@@ -101,6 +108,23 @@ public function process(ContainerBuilder $container)
         $this->assertRequiredBundlesAreEnabled($container);
     }
 
+    private static function hasDoctrineConnectionsConfigured(array $configs): bool
+    {
+        foreach ($configs as $config) {
+            if (!isset($config['dbal'])) {
+                continue;
+            }
+
+            if (isset($config['dbal']['connections'])
+                && \count($config['dbal']['connections']) > 0
+            ) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
     private function assertRequiredBundlesAreEnabled(ContainerBuilder $container): void
     {
         $requiredBundles = [
@@ -218,7 +242,7 @@ private function configurePersistence(LoaderInterface $loader, ContainerBuilder
         }
 
         $persistenceConfig = current($config['persistence']);
-        $persistenceMethod = key($config['persistence']);
+        $persistenceMethod = $this->getPersistenceMethod($config);
 
         switch ($persistenceMethod) {
             case 'in_memory':
@@ -232,6 +256,13 @@ private function configurePersistence(LoaderInterface $loader, ContainerBuilder
         }
     }
 
+    private function getPersistenceMethod(array $config): ?string
+    {
+        $persistenceMethod = key($config['persistence']);
+
+        return \is_string($persistenceMethod) ? $persistenceMethod : null;
+    }
+
     private function configureDoctrinePersistence(ContainerBuilder $container, array $config, array $persistenceConfig): void
     {
         $entityManagerName = $persistenceConfig['entity_manager'];