diff --git a/src/Recurr/DateUtil.php b/src/Recurr/DateUtil.php
index 48a4e87..d53ed5e 100644
--- a/src/Recurr/DateUtil.php
+++ b/src/Recurr/DateUtil.php
@@ -111,8 +111,8 @@ public static function getDateInfo(\DateTimeInterface $dt)
     public static function getDaySetOfWeek(
         \DateTimeInterface $dt,
         \DateTimeInterface $start,
-        Rule $rule = null,
-        DateInfo $dtInfo = null
+        ?Rule $rule = null,
+        ?DateInfo $dtInfo = null
     )
     {
         $start = clone $dt;
diff --git a/src/Recurr/Recurrence.php b/src/Recurr/Recurrence.php
index 48ab117..736d3bb 100644
--- a/src/Recurr/Recurrence.php
+++ b/src/Recurr/Recurrence.php
@@ -27,7 +27,7 @@ class Recurrence
     /** @var int */
     protected $index;
 
-    public function __construct(\DateTimeInterface $start = null, \DateTimeInterface $end = null, $index = 0)
+    public function __construct(?\DateTimeInterface $start = null, ?\DateTimeInterface $end = null, $index = 0)
     {
         if ($start instanceof \DateTimeInterface) {
             $this->setStart($start);
diff --git a/src/Recurr/Transformer/ArrayTransformer.php b/src/Recurr/Transformer/ArrayTransformer.php
index 414a3e6..06a5026 100644
--- a/src/Recurr/Transformer/ArrayTransformer.php
+++ b/src/Recurr/Transformer/ArrayTransformer.php
@@ -52,7 +52,7 @@ class ArrayTransformer
      *
      * @param ArrayTransformerConfig $config
      */
-    public function __construct(ArrayTransformerConfig $config = null)
+    public function __construct(?ArrayTransformerConfig $config = null)
     {
         if (!$config instanceof ArrayTransformerConfig) {
             $config = new ArrayTransformerConfig();
@@ -83,7 +83,7 @@ public function setConfig($config)
      * @return RecurrenceCollection|Recurrence[]
      * @throws InvalidWeekday
      */
-    public function transform(Rule $rule, ConstraintInterface $constraint = null, $countConstraintFailures = true)
+    public function transform(Rule $rule, ?ConstraintInterface $constraint = null, $countConstraintFailures = true)
     {
         $start = $rule->getStartDate();
         $end   = $rule->getEndDate();
diff --git a/src/Recurr/Transformer/TextTransformer.php b/src/Recurr/Transformer/TextTransformer.php
index 0fdef0b..2b7dd2d 100644
--- a/src/Recurr/Transformer/TextTransformer.php
+++ b/src/Recurr/Transformer/TextTransformer.php
@@ -9,7 +9,7 @@ class TextTransformer
     protected $fragments = array();
     protected $translator;
 
-    public function __construct(TranslatorInterface $translator = null)
+    public function __construct(?TranslatorInterface $translator = null)
     {
         $this->translator = $translator ?: new Translator('en');
     }