diff --git a/src/Models/Entry.php b/src/Models/Entry.php
index 9a6c4be03aac5fdf7fe1968168d0b1e4b25642a6..6c3546a93f50ead45eef153b02244a3f76449ebd 100644
--- a/src/Models/Entry.php
+++ b/src/Models/Entry.php
@@ -98,7 +98,7 @@ class Entry extends Model implements EntryContract
      * @param  Model|null  $model
      * @return $this
      */
-    public function by(Model $model = null)
+    public function by(?Model $model = null)
     {
         $this->participant()->associate($model);
 
diff --git a/src/Models/Survey.php b/src/Models/Survey.php
index a2d92a4a705d05eaf66e43f15242d649ec3ecb20..58dfcafdd630a08e58a2653ca33f00a57711c21e 100644
--- a/src/Models/Survey.php
+++ b/src/Models/Survey.php
@@ -113,7 +113,7 @@ class Survey extends Model implements SurveyContract
      * @param  Model  $participant
      * @return \Illuminate\Database\Eloquent\Relations\HasMany
      */
-    public function lastEntry(Model $participant = null)
+    public function lastEntry(?Model $participant = null)
     {
         return $participant === null ? null : $this->entriesFrom($participant)->first();
     }
@@ -124,7 +124,7 @@ class Survey extends Model implements SurveyContract
      * @param  Model|null  $model
      * @return bool
      */
-    public function isEligible(Model $participant = null)
+    public function isEligible(?Model $participant = null)
     {
         if ($participant === null) {
             return $this->acceptsGuestEntries();
diff --git a/tests/TestCase.php b/tests/TestCase.php
index a62cf6903879dd84097ba79e0d76ffaf458694f7..35468da7265dd048130e5df294e46f07a0057410 100644
--- a/tests/TestCase.php
+++ b/tests/TestCase.php
@@ -62,7 +62,7 @@ class TestCase extends Orchestra
      * @param  User|null  $user
      * @return User
      */
-    protected function signIn(User $user = null)
+    protected function signIn(?User $user = null)
     {
         $user = $user ?? User::forceCreate([
             'name' => 'John',