diff --git a/composer.json b/composer.json
index c1f4afe..27bad9f 100644
--- a/composer.json
+++ b/composer.json
@@ -16,9 +16,9 @@
   "require-dev": {
     "doctrine/coding-standard": "^5.0",
     "phpbench/phpbench": "^0.14.0",
-    "phpstan/phpstan": "^0.10.3",
-    "phpstan/phpstan-phpunit": "^0.10.0",
-    "phpstan/phpstan-strict-rules": "^0.10.1",
+    "phpstan/phpstan": "0.10.5",
+    "phpstan/phpstan-phpunit": "0.10.0",
+    "phpstan/phpstan-strict-rules": "0.10.1",
     "phpunit/phpunit": "^7.2",
     "psr/http-message": "^1.0",
     "react/promise": "2.*"
diff --git a/phpcs.xml.dist b/phpcs.xml.dist
index aba1cb2..803dd12 100644
--- a/phpcs.xml.dist
+++ b/phpcs.xml.dist
@@ -13,10 +13,12 @@
     <file>tests</file>
 
     <rule ref="Doctrine">
-        <!--Disable PHP7+ features that might cause BC breaks for now -->
+        <!-- Disable PHP7+ features that might cause BC breaks for now -->
         <exclude name="SlevomatCodingStandard.Classes.ClassConstantVisibility.MissingConstantVisibility" />
         <exclude name="SlevomatCodingStandard.TypeHints.TypeHintDeclaration.MissingParameterTypeHint" />
         <exclude name="SlevomatCodingStandard.TypeHints.TypeHintDeclaration.MissingReturnTypeHint" />
+        <!-- Enable when Slevomat starts supporting variadics, see https://github.com/slevomat/coding-standard/issues/251 -->
+        <exclude name="SlevomatCodingStandard.Namespaces.UnusedUses.UnusedUse" />
     </rule>
 
     <!--@api annotation is required for now -->
diff --git a/src/Executor/Promise/Adapter/SyncPromise.php b/src/Executor/Promise/Adapter/SyncPromise.php
index 4d1f96e..91e139c 100644
--- a/src/Executor/Promise/Adapter/SyncPromise.php
+++ b/src/Executor/Promise/Adapter/SyncPromise.php
@@ -123,7 +123,7 @@ class SyncPromise
 
                 if ($this->state === self::FULFILLED) {
                     try {
-                        $promise->resolve($onFulfilled ? $onFulfilled($this->result) : $this->result);
+                        $promise->resolve($onFulfilled === null ? $this->result : $onFulfilled($this->result));
                     } catch (Exception $e) {
                         $promise->reject($e);
                     } catch (Throwable $e) {
@@ -131,10 +131,10 @@ class SyncPromise
                     }
                 } elseif ($this->state === self::REJECTED) {
                     try {
-                        if ($onRejected) {
-                            $promise->resolve($onRejected($this->result));
-                        } else {
+                        if ($onRejected === null) {
                             $promise->reject($this->result);
+                        } else {
+                            $promise->resolve($onRejected($this->result));
                         }
                     } catch (Exception $e) {
                         $promise->reject($e);
diff --git a/src/Type/Definition/EnumType.php b/src/Type/Definition/EnumType.php
index aa5c8b4..2b42590 100644
--- a/src/Type/Definition/EnumType.php
+++ b/src/Type/Definition/EnumType.php
@@ -188,7 +188,7 @@ class EnumType extends Type implements InputType, OutputType, LeafType, NamedTyp
             $lookup = $this->getNameLookup();
             if (isset($lookup[$valueNode->value])) {
                 $enumValue = $lookup[$valueNode->value];
-                if ($enumValue) {
+                if ($enumValue !== null) {
                     return $enumValue->value;
                 }
             }
diff --git a/src/Utils/TypeInfo.php b/src/Utils/TypeInfo.php
index 722195d..430e7ad 100644
--- a/src/Utils/TypeInfo.php
+++ b/src/Utils/TypeInfo.php
@@ -22,6 +22,7 @@ use GraphQL\Type\Definition\InputType;
 use GraphQL\Type\Definition\InterfaceType;
 use GraphQL\Type\Definition\ListOfType;
 use GraphQL\Type\Definition\ObjectType;
+use GraphQL\Type\Definition\OutputType;
 use GraphQL\Type\Definition\Type;
 use GraphQL\Type\Definition\UnionType;
 use GraphQL\Type\Definition\WrappingType;
diff --git a/src/Validator/Rules/OverlappingFieldsCanBeMerged.php b/src/Validator/Rules/OverlappingFieldsCanBeMerged.php
index d872245..8dfa7f1 100644
--- a/src/Validator/Rules/OverlappingFieldsCanBeMerged.php
+++ b/src/Validator/Rules/OverlappingFieldsCanBeMerged.php
@@ -366,8 +366,8 @@ class OverlappingFieldsCanBeMerged extends ValidationRule
             );
 
         // The return type for each field.
-        $type1 = $def1 ? $def1->getType() : null;
-        $type2 = $def2 ? $def2->getType() : null;
+        $type1 = $def1 === null ? null : $def1->getType();
+        $type2 = $def2 === null ? null : $def2->getType();
 
         if (! $areMutuallyExclusive) {
             // Two aliases must refer to the same field.