diff --git a/DependencyInjection/Configuration.php b/DependencyInjection/Configuration.php
index 896604c..d44ebef 100644
--- a/DependencyInjection/Configuration.php
+++ b/DependencyInjection/Configuration.php
@@ -75,9 +75,9 @@ class Configuration
                                 ->children()
                                     ->arrayNode('persistence')
                                         ->validate()
-                                            ->ifTrue(function($v) { return 'propel' === $v['driver'] && isset($v['listener']); })
+                                            ->ifTrue(function($v) { return isset($v['driver']) && 'propel' === $v['driver'] && isset($v['listener']); })
                                             ->thenInvalid('Propel doesn\'t support listeners')
-                                            ->ifTrue(function($v) { return 'propel' === $v['driver'] && isset($v['repository']); })
+                                            ->ifTrue(function($v) { return isset($v['driver']) && 'propel' === $v['driver'] && isset($v['repository']); })
                                             ->thenInvalid('Propel doesn\'t support the "repository" parameter')
                                         ->end()
                                         ->children()
@@ -150,9 +150,9 @@ class Configuration
                 ->children()
                     ->arrayNode('persistence')
                         ->validate()
-                            ->ifTrue(function($v) { return 'propel' === $v['driver'] && isset($v['listener']); })
+                            ->ifTrue(function($v) { return isset($v['driver']) && 'propel' === $v['driver'] && isset($v['listener']); })
                             ->thenInvalid('Propel doesn\'t support listeners')
-                            ->ifTrue(function($v) { return 'propel' === $v['driver'] && isset($v['repository']); })
+                            ->ifTrue(function($v) { return isset($v['driver']) && 'propel' === $v['driver'] && isset($v['repository']); })
                             ->thenInvalid('Propel doesn\'t support the "repository" parameter')
                         ->end()
                         ->children()