diff --git a/extensions/mentions/src/Formatter/UnparsePostMentions.php b/extensions/mentions/src/Formatter/UnparsePostMentions.php
index f223dd638f..3cfbfa0558 100644
--- a/extensions/mentions/src/Formatter/UnparsePostMentions.php
+++ b/extensions/mentions/src/Formatter/UnparsePostMentions.php
@@ -28,12 +28,16 @@ public function __construct(TranslatorInterface $translator)
     /**
      * Configure rendering for user mentions.
      *
-     * @param string $xml
+     * @param string|null $xml
      * @param mixed $context
-     * @return string $xml to be unparsed
+     * @return mixed $xml to be unparsed
      */
-    public function __invoke($context, string $xml)
+    public function __invoke($context, $xml)
     {
+        if ($xml === null) {
+            return $xml;
+        }
+
         $xml = $this->updatePostMentionTags($context, $xml);
         $xml = $this->unparsePostMentionTags($xml);
 
diff --git a/extensions/mentions/src/Formatter/UnparseTagMentions.php b/extensions/mentions/src/Formatter/UnparseTagMentions.php
index b2cae82f2f..ff3ccf8342 100644
--- a/extensions/mentions/src/Formatter/UnparseTagMentions.php
+++ b/extensions/mentions/src/Formatter/UnparseTagMentions.php
@@ -18,12 +18,16 @@ class UnparseTagMentions
     /**
      * Configure rendering for user mentions.
      *
-     * @param string $xml
+     * @param string|null $xml
      * @param mixed $context
-     * @return string $xml to be unparsed
+     * @return mixed $xml to be unparsed
      */
-    public function __invoke($context, string $xml)
+    public function __invoke($context, $xml)
     {
+        if ($xml === null) {
+            return $xml;
+        }
+
         $xml = $this->updateTagMentionTags($context, $xml);
         $xml = $this->unparseTagMentionTags($xml);
 
diff --git a/extensions/mentions/src/Formatter/UnparseUserMentions.php b/extensions/mentions/src/Formatter/UnparseUserMentions.php
index b150d2f7ba..f42c7bac39 100644
--- a/extensions/mentions/src/Formatter/UnparseUserMentions.php
+++ b/extensions/mentions/src/Formatter/UnparseUserMentions.php
@@ -29,12 +29,16 @@ public function __construct(TranslatorInterface $translator)
     /**
      * Configure rendering for user mentions.
      *
-     * @param string $xml
+     * @param string|null $xml
      * @param mixed $context
-     * @return string $xml to be unparsed
+     * @return mixed $xml to be unparsed
      */
-    public function __invoke($context, string $xml)
+    public function __invoke($context, $xml)
     {
+        if ($xml === null) {
+            return $xml;
+        }
+
         $xml = $this->updateUserMentionTags($context, $xml);
         $xml = $this->unparseUserMentionTags($xml);
 
diff --git a/extensions/mentions/tests/integration/api/CreateDiscussionTest.php b/extensions/mentions/tests/integration/api/CreateDiscussionTest.php
new file mode 100644
index 0000000000..6c93a34e49
--- /dev/null
+++ b/extensions/mentions/tests/integration/api/CreateDiscussionTest.php
@@ -0,0 +1,137 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Mentions\Tests\integration\api;
+
+use Flarum\Extend;
+use Flarum\Testing\integration\TestCase;
+
+class CreateDiscussionTest extends TestCase
+{
+    /**
+     * @inheritDoc
+     */
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        $this->extension('flarum-mentions');
+
+        $this->extend(
+            (new Extend\Event())
+            ->listen(\Flarum\Post\Event\Saving::class, function ($event) {
+                $event->post->content;
+            })
+        );
+    }
+
+    /**
+     * @test
+     */
+    public function cannot_create_discussion_with_empty_string()
+    {
+        $response = $this->send(
+            $this->request('POST', '/api/discussions', [
+                'authenticatedAs' => 1,
+                'json' => [
+                    'data' => [
+                        'attributes' => [
+                            'title' => 'Test post',
+                            'content' => '',
+                        ],
+                    ],
+                ],
+            ])
+        );
+
+        $this->assertEquals(422, $response->getStatusCode());
+
+        $body = (string) $response->getBody();
+        $this->assertJson($body);
+        $this->assertEquals([
+            'errors' => [
+                [
+                    'status' => '422',
+                    'code' => 'validation_error',
+                    'detail' => 'The content field is required.',
+                    'source' => ['pointer' => '/data/attributes/content'],
+                ],
+            ],
+        ], json_decode($body, true));
+    }
+
+    /**
+     * @test
+     */
+    public function cannot_create_discussion_without_content_property()
+    {
+        $response = $this->send(
+            $this->request('POST', '/api/discussions', [
+                'authenticatedAs' => 1,
+                'json' => [
+                    'data' => [
+                        'attributes' => [
+                            'title' => 'Test post',
+                        ],
+                    ],
+                ],
+            ])
+        );
+
+        $this->assertEquals(422, $response->getStatusCode());
+
+        $body = (string) $response->getBody();
+        $this->assertJson($body);
+        $this->assertEquals([
+            'errors' => [
+                [
+                    'status' => '422',
+                    'code' => 'validation_error',
+                    'detail' => 'The content field is required.',
+                    'source' => ['pointer' => '/data/attributes/content'],
+                ],
+            ],
+        ], json_decode($body, true));
+    }
+
+    /**
+     * @test
+     */
+    public function cannot_create_discussion_with_content_set_to_null()
+    {
+        $response = $this->send(
+            $this->request('POST', '/api/discussions', [
+                'authenticatedAs' => 1,
+                'json' => [
+                    'data' => [
+                        'attributes' => [
+                            'title' => 'Test post',
+                            'content' => null,
+                        ],
+                    ],
+                ],
+            ])
+        );
+
+        $this->assertEquals(422, $response->getStatusCode());
+
+        $body = (string) $response->getBody();
+        $this->assertJson($body);
+        $this->assertEquals([
+            'errors' => [
+                [
+                    'status' => '422',
+                    'code' => 'validation_error',
+                    'detail' => 'The content field is required.',
+                    'source' => ['pointer' => '/data/attributes/content'],
+                ],
+            ],
+        ], json_decode($body, true));
+    }
+}
diff --git a/extensions/mentions/tests/integration/api/EditPostTest.php b/extensions/mentions/tests/integration/api/EditPostTest.php
new file mode 100644
index 0000000000..52b48a1f74
--- /dev/null
+++ b/extensions/mentions/tests/integration/api/EditPostTest.php
@@ -0,0 +1,110 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Mentions\Tests\integration\api;
+
+use Flarum\Extend;
+use Flarum\Testing\integration\TestCase;
+
+class EditPostTest extends TestCase
+{
+    /**
+     * @inheritDoc
+     */
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        $this->extension('flarum-mentions');
+
+        $this->prepareDatabase([
+            'discussions' => [
+                ['id' => 1, 'title' => 'Discussion with post', 'user_id' => 1, 'first_post_id' => 1, 'comment_count' => 1],
+            ],
+            'posts' => [
+                ['id' => 1, 'discussion_id' => 1, 'user_id' => 1, 'type' => 'comment', 'content' => '<t><p>Text</p></t>'],
+            ]
+        ]);
+
+        $this->extend(
+            (new Extend\Event())
+            ->listen(\Flarum\Post\Event\Saving::class, function ($event) {
+                $event->post->content;
+            })
+        );
+    }
+
+    /**
+     * @test
+     */
+    public function cannot_update_post_with_empty_string()
+    {
+        $response = $this->send(
+            $this->request('PATCH', '/api/posts/1', [
+                'authenticatedAs' => 1,
+                'json' => [
+                    'data' => [
+                        'attributes' => [
+                            'content' => '',
+                        ],
+                    ],
+                ],
+            ])
+        );
+
+        $this->assertEquals(422, $response->getStatusCode());
+
+        $body = (string) $response->getBody();
+        $this->assertJson($body);
+        $this->assertEquals([
+            'errors' => [
+                [
+                    'status' => '422',
+                    'code' => 'validation_error',
+                    'detail' => 'The content field is required.',
+                    'source' => ['pointer' => '/data/attributes/content'],
+                ],
+            ],
+        ], json_decode($body, true));
+    }
+
+    /**
+     * @test
+     */
+    public function cannot_update_post_with_invalid_content_type()
+    {
+        $response = $this->send(
+            $this->request('PATCH', '/api/posts/1', [
+                'authenticatedAs' => 1,
+                'json' => [
+                    'data' => [
+                        'attributes' => [
+                            'content' => [],
+                        ],
+                    ],
+                ],
+            ])
+        );
+
+        $this->assertEquals(422, $response->getStatusCode());
+
+        $body = (string) $response->getBody();
+        $this->assertJson($body);
+        $this->assertEquals([
+            'errors' => [
+                [
+                    'status' => '422',
+                    'code' => 'validation_error',
+                    'detail' => 'The content field is required.',
+                    'source' => ['pointer' => '/data/attributes/content'],
+                ],
+            ],
+        ], json_decode($body, true));
+    }
+}
diff --git a/framework/core/src/Formatter/Formatter.php b/framework/core/src/Formatter/Formatter.php
index f47daae694..b93072eddf 100644
--- a/framework/core/src/Formatter/Formatter.php
+++ b/framework/core/src/Formatter/Formatter.php
@@ -139,7 +139,7 @@ public function unparse($xml, $context = null)
             $xml = $callback($context, $xml);
         }
 
-        return Unparser::unparse($xml);
+        return $xml !== null ? Unparser::unparse($xml) : null;
     }
 
     /**