diff --git a/DependencyInjection/Factory/Resolver/AwsS3ResolverFactory.php b/DependencyInjection/Factory/Resolver/AwsS3ResolverFactory.php
index 29976cd3..75991911 100644
--- a/DependencyInjection/Factory/Resolver/AwsS3ResolverFactory.php
+++ b/DependencyInjection/Factory/Resolver/AwsS3ResolverFactory.php
@@ -92,7 +92,7 @@ public function addConfiguration(ArrayNodeDefinition $builder)
                     ->cannotBeEmpty()
                 ->end()
                 ->scalarNode('cache_prefix')
-                    ->defaultValue(null)
+                    ->defaultValue('')
                 ->end()
                 ->arrayNode('client_config')
                     ->isRequired()
diff --git a/DependencyInjection/Factory/Resolver/FlysystemResolverFactory.php b/DependencyInjection/Factory/Resolver/FlysystemResolverFactory.php
index 76e542bc..f6090aba 100644
--- a/DependencyInjection/Factory/Resolver/FlysystemResolverFactory.php
+++ b/DependencyInjection/Factory/Resolver/FlysystemResolverFactory.php
@@ -49,7 +49,7 @@ public function addConfiguration(ArrayNodeDefinition $builder)
                     ->cannotBeEmpty()
                 ->end()
                 ->scalarNode('cache_prefix')
-                    ->defaultValue(null)
+                    ->defaultValue('')
                 ->end()
                 ->scalarNode('root_url')
                     ->isRequired()