diff --git a/src/@types/i18next.d.ts b/src/@types/i18next.d.ts
new file mode 100644
index 00000000..9f98f292
--- /dev/null
+++ b/src/@types/i18next.d.ts
@@ -0,0 +1,28 @@
+/*
+Copyright 2023 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import "i18next";
+// import all namespaces (for the default language, only)
+import app from "../../public/locales/en-GB/app.json";
+
+declare module "i18next" {
+  interface CustomTypeOptions {
+    defaultNS: "app";
+    resources: {
+      app: typeof app;
+    };
+  }
+}
diff --git a/src/TranslatedError.ts b/src/TranslatedError.ts
index 6c7f5dad..a9bd2e1d 100644
--- a/src/TranslatedError.ts
+++ b/src/TranslatedError.ts
@@ -16,6 +16,9 @@ limitations under the License.
 
 import i18n from "i18next";
 
+import type { ParseKeys, TFunction } from "i18next/typescript/t";
+import type { DefaultNamespace, TOptions } from "i18next/typescript/options";
+
 /**
  * An error with messages in both English and the user's preferred language.
  */
@@ -27,8 +30,11 @@ export abstract class TranslatedError extends Error {
    */
   public readonly translatedMessage: string;
 
-  public constructor(messageKey: string, translationFn: typeof i18n.t) {
-    super(translationFn(messageKey, { lng: "en-GB" }));
+  public constructor(
+    messageKey: ParseKeys<DefaultNamespace, TOptions>,
+    translationFn: TFunction<DefaultNamespace>,
+  ) {
+    super(translationFn(messageKey, { lng: "en-GB" } as TOptions));
     this.translatedMessage = translationFn(messageKey);
   }
 }
@@ -38,6 +44,6 @@ class TranslatedErrorImpl extends TranslatedError {}
 // i18next-parser can't detect calls to a constructor, so we expose a bare
 // function instead
 export const translatedError = (
-  messageKey: string,
+  messageKey: ParseKeys<DefaultNamespace, TOptions>,
   t: typeof i18n.t,
 ): TranslatedError => new TranslatedErrorImpl(messageKey, t);
diff --git a/tsconfig.json b/tsconfig.json
index 961199a9..2544804f 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -14,6 +14,7 @@
     "noUnusedLocals": true,
     "moduleResolution": "node",
     "declaration": true,
+    "resolveJsonModule": true,
 
     // TODO: Enable the following options later.
     // "forceConsistentCasingInFileNames": true,