diff --git a/src/server/i18n/index.test.ts b/src/server/i18n/index.test.ts
index c52ce88bf..550215736 100644
--- a/src/server/i18n/index.test.ts
+++ b/src/server/i18n/index.test.ts
@@ -36,6 +36,7 @@ describe('Runner i18n', () => {
     it('returns the language set in the session', () => {
       const blankRequest = {
         yar: {
+          id: 'session-id',
           get: jest.fn().mockReturnValue('cy')
         }
       } as unknown as Request
@@ -46,6 +47,7 @@ describe('Runner i18n', () => {
       const mockYarSet = jest.fn()
       const blankRequest = {
         yar: {
+          id: 'session-id',
           get: jest.fn(),
           set: mockYarSet
         },
diff --git a/src/server/utils/utils.js b/src/server/utils/utils.js
index 823078f52..feea0847e 100644
--- a/src/server/utils/utils.js
+++ b/src/server/utils/utils.js
@@ -43,11 +43,16 @@ export function resolveLanguage(query, yar) {
 
   query ??= {}
 
-  if (yar && 'language' in query) {
+  // yar.id is only set once yar's session has been initialised (onPreAuth),
+  // so calling yar.get/set beforehand (e.g. for a 404 raised pre-routing)
+  // would throw as its internal store is still null
+  const sessionReady = yar && Boolean(yar.id)
+
+  if (sessionReady && 'language' in query) {
     yar.set('language', query.language)
   }
 
-  return yar?.get('language') ?? defaultLang
+  return (sessionReady ? yar.get('language') : null) ?? defaultLang
 }
 
 /**
diff --git a/src/server/utils/utils.test.js b/src/server/utils/utils.test.js
index 23d528c56..ae787296b 100644
--- a/src/server/utils/utils.test.js
+++ b/src/server/utils/utils.test.js
@@ -3,7 +3,8 @@ import { getTraceId } from '@defra/hapi-tracing'
 import { config } from '~/src/config/index.js'
 import {
   applyTraceHeaders,
-  getFeedbackFormLink
+  getFeedbackFormLink,
+  resolveLanguage
 } from '~/src/server/utils/utils.js'
 
 jest.mock('@defra/hapi-tracing')
@@ -64,4 +65,41 @@ describe('utils', () => {
       })
     })
   })
+
+  describe('resolveLanguage', () => {
+    const mockYarStore = /** @type {Record<string,string>} */ ({})
+    /**
+     * @param {string} name
+     * @param {string} value
+     */
+    function yarSet(name, value) {
+      mockYarStore[name] = value
+    }
+    /**
+     * @param {string} name
+     */
+    function yarGet(name) {
+      return mockYarStore[name]
+    }
+
+    it('should return default language if yar session not properly constructed yet', () => {
+      expect(resolveLanguage({}, undefined)).toBe('en-GB')
+    })
+
+    it('should return specified language if yar session up and language passed as query param', () => {
+      expect(
+        resolveLanguage(
+          { language: 'cy' },
+          // @ts-expect-error - partial mock of methods
+          { id: 'session-id', set: yarSet, get: yarGet }
+        )
+      ).toBe('cy')
+
+      // Should retrieve session value when language not passed
+      expect(
+        // @ts-expect-error - partial mock of methods
+        resolveLanguage({}, { id: 'session-id', set: yarSet, get: yarGet })
+      ).toBe('cy')
+    })
+  })
 })