diff --git a/.travis.yml b/.travis.yml
index 9812240..67e67e2 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,10 +2,9 @@ language: php
 
 php:
     - 7.1
+    - nightly
 
 matrix:
-    include:
-        - php: nightly
     allow_failures:
         - php: nightly
     fast_finish: true
diff --git a/src/ServiceContainer/SymfonyExtension.php b/src/ServiceContainer/SymfonyExtension.php
index 7eb8b60..0c08971 100644
--- a/src/ServiceContainer/SymfonyExtension.php
+++ b/src/ServiceContainer/SymfonyExtension.php
@@ -277,7 +277,7 @@ final class SymfonyExtension implements Extension
      *
      * @throws \DomainException
      */
-    private function requireKernelBootstrapFile(string $basePath, string $bootstrapPath): void
+    private function requireKernelBootstrapFile(string $basePath, ?string $bootstrapPath): void
     {
         if (null === $bootstrapPath) {
             return;