diff --git a/src/Lunr/Vortex/FCM/FCMPayload.php b/src/Lunr/Vortex/FCM/FCMPayload.php
index bfbc2e1..a069308 100644
--- a/src/Lunr/Vortex/FCM/FCMPayload.php
+++ b/src/Lunr/Vortex/FCM/FCMPayload.php
@@ -10,6 +10,8 @@
 
 namespace Lunr\Vortex\FCM;
 
+use InvalidArgumentException;
+
 /**
  * Firebase Cloud Messaging Push Notification Payload Generator.
  */
@@ -89,6 +91,17 @@ public function get_json_payload(int $flag = 0): string
      */
     public function set_data(array $data): static
     {
+        foreach ($data as $key => &$value)
+        {
+            // @phpstan-ignore-next-line
+            if (is_string($value) === FALSE)
+            {
+                throw new InvalidArgumentException('Data type of ' . $key . ' must be a string!');
+            }
+        }
+
+        unset($value);
+
         $this->elements['data'] = $data;
 
         return $this;
diff --git a/src/Lunr/Vortex/FCM/Tests/FCMPayloadSetTest.php b/src/Lunr/Vortex/FCM/Tests/FCMPayloadSetTest.php
index 10701c5..fcf37f6 100644
--- a/src/Lunr/Vortex/FCM/Tests/FCMPayloadSetTest.php
+++ b/src/Lunr/Vortex/FCM/Tests/FCMPayloadSetTest.php
@@ -10,6 +10,8 @@
 
 namespace Lunr\Vortex\FCM\Tests;
 
+use InvalidArgumentException;
+
 /**
  * This class contains tests for the setters of the FCMPayload class.
  *
@@ -43,6 +45,21 @@ public function testSetNotificationReturnsSelfReference(): void
         $this->assertSame($this->class, $this->class->set_notification([]));
     }
 
+    /**
+     * Test set_data() decodes array value to string.
+     *
+     * @covers \Lunr\Vortex\FCM\FCMPayload::set_data
+     */
+    public function testSetDataThrowsExceptionWhenArrayValueIsNotString(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Data type of test_key must be a string!');
+
+        $this->class->set_data([ 'test_key' => [ 'value_key' => 'value' ] ]);
+
+        $value = $this->get_reflection_property_value('elements');
+    }
+
     /**
      * Test set_data() works correctly.
      *
@@ -65,7 +82,7 @@ public function testSetData(): void
      */
     public function testSetDataReturnsSelfReference(): void
     {
-        $this->assertSame($this->class, $this->class->set_data([]));
+        $this->assertSame($this->class, $this->class->set_data([ 'key' => 'value' ]));
     }
 
     /**