diff --git a/.github/workflows/pull-request-checks.yaml b/.github/workflows/pull-request-checks.yaml
index 0c97a4a..2aba5e1 100644
--- a/.github/workflows/pull-request-checks.yaml
+++ b/.github/workflows/pull-request-checks.yaml
@@ -30,12 +30,13 @@ jobs:
       - name: Run Composer linter
         run: composer validate --strict --no-check-lock
 
+      - name: Run PHP unit test suite
+        run: composer run-script test
+
       - name: Run phpstan
         run: composer run-script phpstan
 
       - name: Run lint PHP
         run: composer run-script php-lint
 
-      # Tests
-      - name: Run PHP unit test suite
-        run: composer run-script test
+
diff --git a/src/Email/MessageOptions.php b/src/Email/MessageOptions.php
index 2c5e23b..1844ecc 100644
--- a/src/Email/MessageOptions.php
+++ b/src/Email/MessageOptions.php
@@ -42,8 +42,12 @@ public function __construct(
         $this->participants = $participants;
     }
 
-    public function getSubject(): ?string
+    public function getSubject(): string
     {
+        if (!\is_string($this->subject)) {
+            throw new InvalidArgumentException('Invalid subject');
+        }
+
         return $this->subject;
     }
 
diff --git a/src/Email/Participants.php b/src/Email/Participants.php
index 0ee4a79..d6b0c74 100644
--- a/src/Email/Participants.php
+++ b/src/Email/Participants.php
@@ -59,8 +59,12 @@ public function __construct(
         $this->bcc = $bcc;
     }
 
-    public function getSender(): ?Address
+    public function getSender(): Address
     {
+        if (!$this->sender instanceof Address) {
+            throw new InvalidArgumentException('Invalid sender');
+        }
+
         return $this->sender;
     }
 
diff --git a/src/Exception/ExceptionInterface.php b/src/Exception/ExceptionInterface.php
new file mode 100644
index 0000000..8c3e551
--- /dev/null
+++ b/src/Exception/ExceptionInterface.php
@@ -0,0 +1,9 @@
+<?php
+
+declare(strict_types=1);
+
+namespace FH\Bundle\MailerBundle\Exception;
+
+interface ExceptionInterface
+{
+}
diff --git a/src/Exception/InvalidArgumentException.php b/src/Exception/InvalidArgumentException.php
new file mode 100644
index 0000000..56bf794
--- /dev/null
+++ b/src/Exception/InvalidArgumentException.php
@@ -0,0 +1,11 @@
+<?php
+
+declare(strict_types=1);
+
+namespace FH\Bundle\MailerBundle\Exception;
+
+use InvalidArgumentException as InvalidArgumentExceptionAlias;
+
+class InvalidArgumentException extends InvalidArgumentExceptionAlias implements ExceptionInterface
+{
+}