diff --git a/src/Formatter/Specialised/BoltOGMTranslator.php b/src/Formatter/Specialised/BoltOGMTranslator.php
index d0788fad..aa3c780b 100644
--- a/src/Formatter/Specialised/BoltOGMTranslator.php
+++ b/src/Formatter/Specialised/BoltOGMTranslator.php
@@ -127,7 +127,10 @@ private function makeFromBoltLocalDateTime(BoltLocalDateTime $time): LocalDateTi
 
     private function makeBoltTimezoneIdentifier(BoltDateTimeZoneId $time): DateTimeZoneId
     {
-        return new DateTimeZoneId($time->seconds(), $time->nanoseconds(), $time->tz_id());
+        /** @var non-empty-string $tzId */
+        $tzId = $time->tz_id();
+
+        return new DateTimeZoneId($time->seconds(), $time->nanoseconds(), $tzId);
     }
 
     private function makeFromBoltDuration(BoltDuration $duration): Duration
diff --git a/src/Types/DateTimeZoneId.php b/src/Types/DateTimeZoneId.php
index 10fcc592..a390c1a7 100644
--- a/src/Types/DateTimeZoneId.php
+++ b/src/Types/DateTimeZoneId.php
@@ -34,6 +34,9 @@
  */
 final class DateTimeZoneId extends AbstractPropertyObject implements BoltConvertibleInterface
 {
+    /**
+     * @param non-empty-string $tzId
+     */
     public function __construct(
         private int $seconds,
         private int $nanoseconds,