diff --git a/packages/frontend/src/os.ts b/packages/frontend/src/os.ts
index 4eb4072b72..97e5e2b1ad 100644
--- a/packages/frontend/src/os.ts
+++ b/packages/frontend/src/os.ts
@@ -57,7 +57,10 @@ export function apiWithDialog<
 		text = initialText.join('\n');
 
 		if ('code' in err && err.code != null) {
-			if (err.code === 'INTERNAL_ERROR') {
+			if (customErrors && customErrors[err.code] != null) {
+				title = customErrors[err.code].title;
+				text = customErrors[err.code].text;
+			} else if (err.code === 'INTERNAL_ERROR') {
 				title = i18n.ts.internalServerError;
 				text = i18n.ts.internalServerErrorDescription;
 				const date = new Date().toISOString();
@@ -93,9 +96,6 @@ export function apiWithDialog<
 			} else if (err.code === 'ROLE_PERMISSION_DENIED') {
 				title = i18n.ts.permissionDeniedError;
 				text = i18n.ts.permissionDeniedErrorDescription;
-			} else if (customErrors && customErrors[err.code] != null) {
-				title = customErrors[err.code].title;
-				text = customErrors[err.code].text;
 			} else if (err.code.startsWith('TOO_MANY')) {
 				title = i18n.ts.youCannotCreateAnymore;
 				if ('id' in err && err.id != null) {
diff --git a/packages/frontend/src/scripts/err-promise.ts b/packages/frontend/src/scripts/err-promise.ts
index 2d334c3b43..24bed5b91b 100644
--- a/packages/frontend/src/scripts/err-promise.ts
+++ b/packages/frontend/src/scripts/err-promise.ts
@@ -3,14 +3,8 @@
  * SPDX-License-Identifier: AGPL-3.0-only
  */
 
-export interface IErrPromise<TSuccess, TError = unknown> {
-	then<TResult1 = TSuccess, TResult2 = never>(onfulfilled?: ((value: TSuccess) => TResult1 | PromiseLike<TResult1>) | undefined | null, onrejected?: ((reason: TError) => TResult2 | PromiseLike<TResult2>) | undefined | null): Promise<TResult1 | TResult2>;
-
-	catch<TResult = never>(onrejected?: ((reason: TError) => TResult | PromiseLike<TResult>) | undefined | null): Promise<TSuccess | TResult>;
-}
-
 /** rejectに型付けができるPromise */
-export class ErrPromise<TSuccess, TError> extends Promise<TSuccess> implements IErrPromise<TSuccess, TError> {
+export class ErrPromise<TSuccess, TError> extends Promise<TSuccess> {
 	constructor(executor: (resolve: (value: TSuccess | PromiseLike<TSuccess>) => void, reject: (reason: TError) => void) => void) {
 		super(executor);
 	}