frontend/src/pages/posts/PostHistoryPage.tsx

@@ -71,7 +71,7 @@ export default (() => {
 			  ++rowsCnt
 		      }
 		    return (
-			<tr key={`${ change.timestamp }-${ change.post.id }-${ change.tag.id }`}
+			<tr key={`${ change.timestamp }-${ change.post.id }-${ change.tag?.id }`}
 			    className={cn ('even:bg-gray-100 dark:even:bg-gray-700',
 					   withPost && 'border-t')}>
 			  {withPost && (
@@ -92,7 +92,9 @@ export default (() => {
 				</PrefetchLink>
 			      </td>)}
 			  <td className="p-2">
-			    <TagLink tag={change.tag} withWiki={false} withCount={false}/>
+			    {change.tag
+			     ? <TagLink tag={change.tag} withWiki={false} withCount={false}/>
+			     : '(マスタ削除済のタグ) '}
 			    {`を${ change.changeType === 'add' ? '記載' : '消除' }`}
 			  </td>
 			  <td className="p-2">

frontend/src/types.ts

@@ -54,8 +54,8 @@ export type Post = {
 
 export type PostTagChange = {
     post:       Post
-    tag:        Tag
+    tag:        Tag | null
-    user?:      User
+    user:       User | null
     changeType: 'add' | 'remove'
     timestamp:  string }