diff --git a/app/static/app.js b/app/static/app.js
index 00f3534..ef788f5 100644
--- a/app/static/app.js
+++ b/app/static/app.js
@@ -51,7 +51,13 @@ async function call(method, path, body) {
     clearTimeout(timeoutId);
   }
   const data = await res.json().catch(() => ({}));
-  if (!res.ok) throw new Error(apiErrorMessage(data.detail) || res.statusText);
+  if (!res.ok) {
+    // A token the server no longer accepts can't be recovered from by retrying:
+    // without this every poll keeps failing against a dashboard that still looks
+    // logged in, toasting "session expired" forever. Drop back to the login form.
+    if (res.status === 401 && data.detail?.code === 'session_expired' && token) logout();
+    throw new Error(apiErrorMessage(data.detail) || res.statusText);
+  }
   return data;
 }
 
@@ -62,6 +68,11 @@ async function call(method, path, body) {
 function apiErrorMessage(detail) {
   if (!detail) return null;
   if (typeof detail === 'string') return detail; // endpoints still returning a bare string
+  // FastAPI's own request-validation failures (422) use a list of field errors
+  // instead, in English and phrased for an API client ("Input should be a valid
+  // integer"). Nothing here can act on which field it was, so say the one useful
+  // thing — the request was malformed — in the user's language.
+  if (Array.isArray(detail)) return t('error.invalid_request');
   return tOrNull('error.' + detail.code, errorParams(detail.params)) || detail.message || null;
 }
 
@@ -702,9 +713,15 @@ async function withdraw() {
   const btn = document.getElementById('withdraw-btn');
   const ext = document.getElementById('wd-address').value;
   const isFullAmount = document.getElementById('wd-full-amount').checked;
-  const amtSats = isFullAmount
-    ? myBalanceSats
-    : Math.round(parseFloat(document.getElementById('wd-amount').value) * SATS_PER_PLM);
+  const amount = parseFloat(document.getElementById('wd-amount').value);
+  // Caught here rather than left to the server: an empty or non-numeric field
+  // parses to NaN, which JSON.stringify sends as null, which comes back as a
+  // 422 whose only readable text is an English HTTP status line.
+  if (!isFullAmount && !(amount > 0)) {
+    toast(t('error.invalid_amount'), 'error');
+    return;
+  }
+  const amtSats = isFullAmount ? myBalanceSats : Math.round(amount * SATS_PER_PLM);
   await withLoading(btn, t('loading.sending'), async () => {
     try {
       await call('POST', '/withdrawals', { external_address: ext, amount_sats: amtSats });
diff --git a/app/static/i18n.js b/app/static/i18n.js
index 6dbe1ad..14fbdc0 100644
--- a/app/static/i18n.js
+++ b/app/static/i18n.js
@@ -130,6 +130,8 @@ const TRANSLATIONS = {
     'error.invalid_credentials': 'Wrong username or password.',
     'error.derivation_index_conflict': 'Registration failed, please try again.',
     'error.session_expired': 'Session expired, please log in again.',
+    'error.invalid_request': 'Invalid request, please check the entered data.',
+    'error.invalid_amount': 'Enter an amount greater than zero.',
 
     'loading.creating': 'Creating…',
     'loading.loggingIn': 'Logging in…',
@@ -260,6 +262,8 @@ const TRANSLATIONS = {
     'error.invalid_credentials': 'Username o password errati.',
     'error.derivation_index_conflict': 'Registrazione non riuscita, riprova.',
     'error.session_expired': 'Sessione scaduta, accedi di nuovo.',
+    'error.invalid_request': 'Richiesta non valida, controlla i dati inseriti.',
+    'error.invalid_amount': 'Inserisci un importo maggiore di zero.',
 
     'loading.creating': 'Creazione…',
     'loading.loggingIn': 'Accesso…',
@@ -390,6 +394,8 @@ const TRANSLATIONS = {
     'error.invalid_credentials': 'Usuario o contraseña incorrectos.',
     'error.derivation_index_conflict': 'No se pudo completar el registro, inténtalo de nuevo.',
     'error.session_expired': 'Sesión caducada, vuelve a iniciar sesión.',
+    'error.invalid_request': 'Solicitud no válida, revisa los datos introducidos.',
+    'error.invalid_amount': 'Introduce un importe mayor que cero.',
 
     'loading.creating': 'Creando…',
     'loading.loggingIn': 'Entrando…',
@@ -520,6 +526,8 @@ const TRANSLATIONS = {
     'error.invalid_credentials': "Nom d'utilisateur ou mot de passe incorrect.",
     'error.derivation_index_conflict': "L'inscription a échoué, veuillez réessayer.",
     'error.session_expired': 'Session expirée, veuillez vous reconnecter.',
+    'error.invalid_request': 'Requête invalide, vérifiez les données saisies.',
+    'error.invalid_amount': 'Saisissez un montant supérieur à zéro.',
 
     'loading.creating': 'Création…',
     'loading.loggingIn': 'Connexion…',
@@ -650,6 +658,8 @@ const TRANSLATIONS = {
     'error.invalid_credentials': 'Benutzername oder Passwort falsch.',
     'error.derivation_index_conflict': 'Registrierung fehlgeschlagen, bitte erneut versuchen.',
     'error.session_expired': 'Sitzung abgelaufen, bitte melde dich erneut an.',
+    'error.invalid_request': 'Ungültige Anfrage, bitte überprüfe die eingegebenen Daten.',
+    'error.invalid_amount': 'Gib einen Betrag größer als null ein.',
 
     'loading.creating': 'Wird erstellt…',
     'loading.loggingIn': 'Anmeldung…',
@@ -780,6 +790,8 @@ const TRANSLATIONS = {
     'error.invalid_credentials': 'Неверное имя пользователя или пароль.',
     'error.derivation_index_conflict': 'Не удалось завершить регистрацию, попробуйте ещё раз.',
     'error.session_expired': 'Сессия истекла, войдите снова.',
+    'error.invalid_request': 'Некорректный запрос, проверьте введённые данные.',
+    'error.invalid_amount': 'Введите сумму больше нуля.',
 
     'loading.creating': 'Создание…',
     'loading.loggingIn': 'Вход…',
@@ -910,6 +922,8 @@ const TRANSLATIONS = {
     'error.invalid_credentials': '用户名或密码错误。',
     'error.derivation_index_conflict': '注册失败，请重试。',
     'error.session_expired': '会话已过期，请重新登录。',
+    'error.invalid_request': '请求无效，请检查填写的内容。',
+    'error.invalid_amount': '请输入大于零的金额。',
 
     'loading.creating': '正在创建…',
     'loading.loggingIn': '正在登录…',