diff --git a/app/MetadataResolver/ActivityPubResolver.php b/app/MetadataResolver/ActivityPubResolver.php
index 7cf3e00..c0580c8 100644
--- a/app/MetadataResolver/ActivityPubResolver.php
+++ b/app/MetadataResolver/ActivityPubResolver.php
@@ -37,6 +37,10 @@ class ActivityPubResolver implements Resolver, Parser
         $activityOrObject = json_decode($json, true);
         $object = $activityOrObject['object'] ?? $activityOrObject;
 
+        if ($object['type'] !== 'Note') {
+            throw new UnsupportedContentException('Unsupported object type: ' . $object['type']);
+        }
+
         $metadata = new Metadata();
 
         $metadata->title = isset($object['attributedTo']) ? $this->getTitleFromActor($object['attributedTo']) : '';
diff --git a/app/MetadataResolver/MetadataResolver.php b/app/MetadataResolver/MetadataResolver.php
index 38539f2..83351db 100644
--- a/app/MetadataResolver/MetadataResolver.php
+++ b/app/MetadataResolver/MetadataResolver.php
@@ -49,16 +49,19 @@ class MetadataResolver implements Resolver
     {
         foreach ($this->rules as $pattern => $class) {
             if (preg_match($pattern, $url) === 1) {
-                /** @var Resolver $resolver */
-                $resolver = app($class);
+                try {
+                    /** @var Resolver $resolver */
+                    $resolver = app($class);
 
-                return $resolver->resolve($url);
+                    return $resolver->resolve($url);
+                } catch (UnsupportedContentException $e) {
+                }
             }
         }
 
-        $result = $this->resolveWithAcceptHeader($url);
-        if ($result !== null) {
-            return $result;
+        try {
+            return $this->resolveWithAcceptHeader($url);
+        } catch (UnsupportedContentException $e) {
         }
 
         if (isset($this->defaultResolver)) {
@@ -71,7 +74,7 @@ class MetadataResolver implements Resolver
         throw new \UnexpectedValueException('URL not matched.');
     }
 
-    public function resolveWithAcceptHeader(string $url): ?Metadata
+    public function resolveWithAcceptHeader(string $url): Metadata
     {
         try {
             // Rails等はAcceptに */* が入っていると、ブラウザの適当なAcceptヘッダだと判断して全部無視してしまう。
@@ -116,6 +119,6 @@ class MetadataResolver implements Resolver
             // 5xx は変なAcceptが原因かもしれない（？）ので無視してフォールバック
         }
 
-        return null;
+        throw new UnsupportedContentException();
     }
 }
diff --git a/app/MetadataResolver/UnsupportedContentException.php b/app/MetadataResolver/UnsupportedContentException.php
new file mode 100644
index 0000000..6ca6ed3
--- /dev/null
+++ b/app/MetadataResolver/UnsupportedContentException.php
@@ -0,0 +1,12 @@
+<?php
+
+namespace App\MetadataResolver;
+
+use Exception;
+
+/**
+ * このResolverやParserが対応していないサイトであったことを表わします。
+ */
+class UnsupportedContentException extends Exception
+{
+}