diff --git a/composer.json b/composer.json
index b688618..918cbca 100644
--- a/composer.json
+++ b/composer.json
@@ -16,7 +16,7 @@
   "require-dev": {
     "doctrine/coding-standard": "^6.0",
     "phpbench/phpbench": "^0.14.0",
-    "phpstan/phpstan": "^0.11.8",
+    "phpstan/phpstan": "^0.11.12",
     "phpstan/phpstan-phpunit": "^0.11.2",
     "phpstan/phpstan-strict-rules": "^0.11.1",
     "phpunit/phpcov": "^5.0",
diff --git a/src/Utils/BuildSchema.php b/src/Utils/BuildSchema.php
index 3039aef..1aa4f68 100644
--- a/src/Utils/BuildSchema.php
+++ b/src/Utils/BuildSchema.php
@@ -159,9 +159,10 @@ class BuildSchema
         // If specified directives were not explicitly declared, add them.
         $skip = array_reduce(
             $directives,
-            static function ($hasSkip, $directive) {
+            static function (bool $hasSkip, Directive $directive) : bool {
                 return $hasSkip || $directive->name === 'skip';
-            }
+            },
+            false
         );
         if (! $skip) {
             $directives[] = Directive::skipDirective();
@@ -169,9 +170,10 @@ class BuildSchema
 
         $include = array_reduce(
             $directives,
-            static function ($hasInclude, $directive) {
+            static function (bool $hasInclude, Directive $directive) : bool {
                 return $hasInclude || $directive->name === 'include';
-            }
+            },
+            false
         );
         if (! $include) {
             $directives[] = Directive::includeDirective();
@@ -179,9 +181,10 @@ class BuildSchema
 
         $deprecated = array_reduce(
             $directives,
-            static function ($hasDeprecated, $directive) {
+            static function (bool $hasDeprecated, Directive $directive) : bool {
                 return $hasDeprecated || $directive->name === 'deprecated';
-            }
+            },
+            false
         );
         if (! $deprecated) {
             $directives[] = Directive::deprecatedDirective();