diff --git a/frontend/src/api/mappers.ts b/frontend/src/api/mappers.ts
index 34e9531..9f3365e 100644
--- a/frontend/src/api/mappers.ts
+++ b/frontend/src/api/mappers.ts
@@ -60,6 +60,17 @@ function readBoolean(record: Record<string, unknown>, key: string, path: string)
   return value;
 }
 
+function readNullableNumber(record: Record<string, unknown>, key: string, path: string): number | null {
+  const value = record[key];
+  if (value === undefined || value === null) {
+    return null;
+  }
+  if (!isNumber(value)) {
+    throw new Error(`Invalid API contract: expected number|null at ${path}.${key}`);
+  }
+  return value;
+}
+
 export function mapHealthResponse(payload: unknown): HealthResponse {
   const root = asRecord(payload, 'health');
   return {
@@ -131,11 +142,11 @@ function mapSessionDetail(payload: unknown): SessionDetailResponse {
       }),
       guesses: guessesRaw.map((guess, index) => {
         const record = asRecord(guess, `session_detail.reveal.guesses[${index}]`);
-        const fooledPlayerIdRaw = record.fooled_player_id;
-        if (fooledPlayerIdRaw !== undefined && fooledPlayerIdRaw !== null && !isNumber(fooledPlayerIdRaw)) {
-          throw new Error(`Invalid API contract: expected number|null at session_detail.reveal.guesses[${index}].fooled_player_id`);
-        }
-        const fooledPlayerId = fooledPlayerIdRaw ?? null;
+        const fooledPlayerId = readNullableNumber(
+          record,
+          'fooled_player_id',
+          `session_detail.reveal.guesses[${index}]`
+        );
         const fooledPlayerNickname = record.fooled_player_nickname;
         if (fooledPlayerNickname !== undefined && !isString(fooledPlayerNickname)) {
           throw new Error(`Invalid API contract: expected string at session_detail.reveal.guesses[${index}].fooled_player_nickname`);
@@ -389,11 +400,7 @@ export function mapSubmitGuessResponse(payload: unknown): SubmitGuessResponse {
   const root = asRecord(payload, 'submit_guess');
   const guess = asRecord(root.guess, 'submit_guess.guess');
   const window = asRecord(root.window, 'submit_guess.window');
-  const fooledPlayerIdRaw = guess.fooled_player_id;
-  if (fooledPlayerIdRaw !== undefined && fooledPlayerIdRaw !== null && !isNumber(fooledPlayerIdRaw)) {
-    throw new Error('Invalid API contract: expected number|null at submit_guess.guess.fooled_player_id');
-  }
-  const fooledPlayerId = fooledPlayerIdRaw ?? null;
+  const fooledPlayerId = readNullableNumber(guess, 'fooled_player_id', 'submit_guess.guess');
 
   return {
     guess: {
diff --git a/frontend/tests/angular-api-client.test.ts b/frontend/tests/angular-api-client.test.ts
index 0fc0b40..584b289 100644
--- a/frontend/tests/angular-api-client.test.ts
+++ b/frontend/tests/angular-api-client.test.ts
@@ -1,6 +1,7 @@
 import { describe, expect, it, vi } from 'vitest';
 
 import { createAngularApiClient, type AngularHttpClientLike } from '../src/api/angular-client';
+import { mapSubmitGuessResponse } from '../src/api/mappers';
 
 describe('createAngularApiClient', () => {
   it('reads health and session detail using Django-compatible endpoints', async () => {
@@ -384,6 +385,22 @@ describe('createAngularApiClient', () => {
     }
   });
 
+  it('maps omitted fooled_player_id to null in submit guess mapper payloads', () => {
+    const mapped = mapSubmitGuessResponse({
+      guess: {
+        id: 200,
+        player_id: 9,
+        round_question_id: 77,
+        selected_text: 'A',
+        is_correct: false,
+        created_at: '2026-03-01T16:01:00Z'
+      },
+      window: { guess_deadline_at: '2026-03-01T16:01:30Z' }
+    });
+
+    expect(mapped.guess.fooled_player_id).toBeNull();
+  });
+
   it('maps host/player gameplay endpoints through typed response mappers', async () => {
     const get = vi.fn<AngularHttpClientLike['get']>(async <T>(url: string) => {
       if (url === '/lobby/sessions/ABCD12/scoreboard') {