diff --git a/CHANGELOG.md b/CHANGELOG.md
index 07d72f7..6ead149 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,13 @@ All notable changes to this project will be documented in this file. This projec
 
 ## Unreleased
 
+## [3.1.1] - 2023-11-20
+
+### Fixed
+
+- [#5](https://github.com/laravel-json-api/encoder-neomerx/issues/5) Fix error with default value for `$jsonApi`
+  property on the `Document` class.
+
 ## [3.1.0] - 2023-11-08
 
 ### Changed
diff --git a/src/Document.php b/src/Document.php
index 166816a..2f7b52f 100644
--- a/src/Document.php
+++ b/src/Document.php
@@ -40,7 +40,7 @@ abstract class Document implements JsonApiDocument
     /**
      * @var JsonApi|null
      */
-    private ?JsonApi $jsonApi;
+    private ?JsonApi $jsonApi = null;
 
     /**
      * @var Links|null