diff --git a/backend/src/common/errors/error-codes.ts b/backend/src/common/errors/error-codes.ts
index 73d7385..79dfe99 100644
--- a/backend/src/common/errors/error-codes.ts
+++ b/backend/src/common/errors/error-codes.ts
@@ -187,6 +187,7 @@ export const ErrorCode = {
   // Voice treatment entry
   VOICE_NOT_AVAILABLE: 'VOICE_NOT_AVAILABLE',
   VOICE_CLIP_TOO_LONG: 'VOICE_CLIP_TOO_LONG',
+  VOICE_UNSUPPORTED_FORMAT: 'VOICE_UNSUPPORTED_FORMAT',
   VOICE_ASR_FAILED: 'VOICE_ASR_FAILED',
   VOICE_EXTRACT_FAILED: 'VOICE_EXTRACT_FAILED',
   VOICE_NOTHING_RECOGNIZED: 'VOICE_NOTHING_RECOGNIZED',
diff --git a/backend/src/modules/voice/dto/voice.dto.ts b/backend/src/modules/voice/dto/voice.dto.ts
index 41b3b40..18a7ca9 100644
--- a/backend/src/modules/voice/dto/voice.dto.ts
+++ b/backend/src/modules/voice/dto/voice.dto.ts
@@ -6,6 +6,7 @@ import {
   MaxLength,
   Min,
 } from 'class-validator';
+import { ErrorCode } from '../../../common/errors/error-codes';
 
 /** Containers OpenRouter's transcription endpoint accepts, and MediaRecorder can produce. */
 export const VOICE_AUDIO_FORMATS = [
@@ -32,10 +33,14 @@ export class ExtractVoiceDto {
    */
   @IsString()
   @IsBase64()
-  @MaxLength(8_000_000)
+  // Both constraints name their own code. Left to the default mapping, `maxLength` falls
+  // through to VALIDATION_FIELD_REQUIRED and `isIn` resolves to
+  // VALIDATION_LANGUAGE_INVALID — so an oversized recording told the clinician a field
+  // was missing, and an unsupported container told them their language was invalid.
+  @MaxLength(8_000_000, { message: ErrorCode.VOICE_CLIP_TOO_LONG })
   audio: string;
 
-  @IsIn(VOICE_AUDIO_FORMATS)
+  @IsIn(VOICE_AUDIO_FORMATS, { message: ErrorCode.VOICE_UNSUPPORTED_FORMAT })
   format: VoiceAudioFormat;
 
   /**
diff --git a/backend/src/modules/voice/due-date.resolver.spec.ts b/backend/src/modules/voice/due-date.resolver.spec.ts
index 5eb5029..74b7167 100644
--- a/backend/src/modules/voice/due-date.resolver.spec.ts
+++ b/backend/src/modules/voice/due-date.resolver.spec.ts
@@ -346,3 +346,40 @@ describe('resolveDueDate', () => {
     });
   });
 });
+
+describe('what an unresolvable deadline quotes back', () => {
+  // The wire shape allows nulls in every field and toVoiceIntent casts rather than
+  // checks, so these reach the resolver intact. The sheet renders `spoken` verbatim.
+  it('never puts "null" or "NaN" in front of the clinician', () => {
+    const bad = [
+      { kind: 'weekday', weekday: null, which: null },
+      { kind: 'offset', unit: null, amount: null },
+      { kind: 'offset', unit: 'day', amount: Number.NaN },
+      { kind: 'jalali', jy: null, jm: 7, jd: 25 },
+      { kind: 'gregorian', y: 2026, m: null, d: null },
+    ];
+    for (const intent of bad) {
+      const result = resolveDueDate(
+        intent as unknown as DueIntent,
+        SATURDAY,
+        FA_WEEK,
+      );
+      expect(result.dueDate).toBeNull();
+      expect(result.unresolved?.spoken ?? '').not.toMatch(/null|NaN/);
+    }
+  });
+
+  it('still quotes a deadline it did understand the words of', () => {
+    const result = resolveDueDate(
+      {
+        kind: 'weekday',
+        weekday: 'thursday',
+        which: null,
+      } as unknown as DueIntent,
+      SATURDAY,
+      FA_WEEK,
+    );
+    // A weekday with no "this/next" resolves, so nothing is quoted back at all.
+    expect(result.dueDate).not.toBeNull();
+  });
+});
diff --git a/backend/src/modules/voice/due-date.resolver.ts b/backend/src/modules/voice/due-date.resolver.ts
index 3411efd..47b8017 100644
--- a/backend/src/modules/voice/due-date.resolver.ts
+++ b/backend/src/modules/voice/due-date.resolver.ts
@@ -77,16 +77,36 @@ function unresolved(spoken: string): DueResolution {
   return { dueDate: null, unresolved: { spoken, reason: 'invalid_date' } };
 }
 
+/**
+ * What to quote back when a deadline could not be resolved.
+ *
+ * Every field here is nullable on the wire and `toVoiceIntent` casts rather than checks,
+ * so a half-classified deadline arrives with nulls in it. The review sheet renders this
+ * verbatim — `"null null" — not a usable date` in front of a clinician is worse than the
+ * reason on its own, which the sheet already handles for a blank string.
+ */
 function describe(intent: DueIntent): string {
+  const usable = (value: unknown): value is number =>
+    typeof value === 'number' && Number.isFinite(value);
+
   switch (intent?.kind) {
     case 'weekday':
-      return `${intent.which} ${intent.weekday}`;
+      // `which` is legitimately null (it means "this"), the weekday is not.
+      return [intent.which, intent.weekday]
+        .filter((part) => typeof part === 'string')
+        .join(' ');
     case 'offset':
-      return `+${intent.amount} ${intent.unit}`;
+      return usable(intent.amount)
+        ? `+${intent.amount} ${intent.unit ?? ''}`.trim()
+        : '';
     case 'jalali':
-      return `${intent.jy}/${intent.jm}/${intent.jd}`;
+      return [intent.jy, intent.jm, intent.jd].every(usable)
+        ? `${intent.jy}/${intent.jm}/${intent.jd}`
+        : '';
     case 'gregorian':
-      return `${intent.y}-${intent.m}-${intent.d}`;
+      return [intent.y, intent.m, intent.d].every(usable)
+        ? `${intent.y}-${intent.m}-${intent.d}`
+        : '';
     default: {
       // Reaching here means an unrecognised `kind`, which resolveDueDate has already
       // established is a string — echo it so the review row names what was heard.
diff --git a/frontend/messages/en.json b/frontend/messages/en.json
index 0c76d3b..f2681c3 100644
--- a/frontend/messages/en.json
+++ b/frontend/messages/en.json
@@ -1267,6 +1267,7 @@
     "VOICE_MIC_DENIED": "Microphone access was blocked. Allow it in your browser settings and try again.",
     "VOICE_NOT_AVAILABLE": "Voice entry is not available for this language yet.",
     "VOICE_CLIP_TOO_LONG": "That recording is too long. Please keep it under two minutes.",
+    "VOICE_UNSUPPORTED_FORMAT": "That recording format is not supported on this device.",
     "VOICE_ASR_FAILED": "Could not turn the recording into text. Please try again.",
     "VOICE_EXTRACT_FAILED": "Could not read the treatment details from the recording.",
     "VOICE_NOTHING_RECOGNIZED": "No speech was recognised. Check the microphone and try again.",
diff --git a/frontend/messages/fa.json b/frontend/messages/fa.json
index f3ec8dd..eb3357a 100644
--- a/frontend/messages/fa.json
+++ b/frontend/messages/fa.json
@@ -1268,6 +1268,7 @@
     "VOICE_MIC_DENIED": "دسترسی به میکروفون مسدود شده است. در تنظیمات مرورگر اجازه دهید و دوباره تلاش کنید.",
     "VOICE_NOT_AVAILABLE": "ثبت گفتاری هنوز برای این زبان در دسترس نیست.",
     "VOICE_CLIP_TOO_LONG": "مدت ضبط بیش از حد است. لطفاً کمتر از دو دقیقه صحبت کنید.",
+    "VOICE_UNSUPPORTED_FORMAT": "قالب این ضبط پشتیبانی نمی‌شود.",
     "VOICE_ASR_FAILED": "تبدیل گفتار به متن انجام نشد. لطفاً دوباره تلاش کنید.",
     "VOICE_EXTRACT_FAILED": "اطلاعات درمان از روی گفتار استخراج نشد.",
     "VOICE_NOTHING_RECOGNIZED": "گفتاری شناسایی نشد. میکروفون را بررسی کنید و دوباره تلاش کنید.",
diff --git a/frontend/messages/nl.json b/frontend/messages/nl.json
index bad8d12..f5f81b8 100644
--- a/frontend/messages/nl.json
+++ b/frontend/messages/nl.json
@@ -1267,6 +1267,7 @@
     "VOICE_MIC_DENIED": "Microfoontoegang is geblokkeerd. Sta dit toe in uw browserinstellingen en probeer opnieuw.",
     "VOICE_NOT_AVAILABLE": "Spraakinvoer is nog niet beschikbaar voor deze taal.",
     "VOICE_CLIP_TOO_LONG": "Die opname is te lang. Houd het onder twee minuten.",
+    "VOICE_UNSUPPORTED_FORMAT": "Dit opnameformaat wordt niet ondersteund.",
     "VOICE_ASR_FAILED": "De opname kon niet naar tekst worden omgezet. Probeer het opnieuw.",
     "VOICE_EXTRACT_FAILED": "De behandelgegevens konden niet uit de opname worden gelezen.",
     "VOICE_NOTHING_RECOGNIZED": "Er is geen spraak herkend. Controleer de microfoon en probeer opnieuw.",