diff --git a/src/2-pages/Review/cards/OutcomeCard.tsx b/src/2-pages/Review/cards/OutcomeCard.tsx
index d346cee0f..0d755683c 100644
--- a/src/2-pages/Review/cards/OutcomeCard.tsx
+++ b/src/2-pages/Review/cards/OutcomeCard.tsx
@@ -1,7 +1,7 @@
 import React, { useState } from 'react'
 import { Box, IconButton, Stack, Typography } from '@mui/material'
 import { formatDate } from '6-shared/helpers/date'
-import { tagModel } from '5-entities/tag'
+import { nullTagId, tagModel } from '5-entities/tag'
 import { DisplayAmount } from '5-entities/currency/displayCurrency'
 import { Card, TCardProps } from '../shared/Card'
 import { useStats } from '../shared/getFacts'
@@ -29,7 +29,7 @@ export function OutcomeCard(props: TCardProps) {
   const { val, tr } = topTransactions[i]
   const { date, comment, payee, tag } = tr
 
-  const tagTitle = tags[tag?.[0] || 'null'].title
+  const tagTitle = (tags[tag?.[0] || nullTagId] ?? tags[nullTagId])?.title
   let additionalInfo = [formatDate(date)]
   if (tagTitle) additionalInfo.push(tagTitle)
   if (payee) additionalInfo.push(payee)
diff --git a/src/2-pages/Review/shared/getFacts.ts b/src/2-pages/Review/shared/getFacts.ts
index 69a9cb842..8b8f739e1 100644
--- a/src/2-pages/Review/shared/getFacts.ts
+++ b/src/2-pages/Review/shared/getFacts.ts
@@ -14,6 +14,7 @@ import { instrumentModel } from '5-entities/currency/instrument'
 import { addFxAmount, convertFx } from '6-shared/helpers/money'
 import { accountModel } from '5-entities/account'
 import { merchantModel } from '5-entities/merchant'
+import { nullTagId } from '5-entities/tag'
 import { TSelector, useAppSelector } from 'store/index'
 
 type TInfoNode = {
@@ -72,7 +73,7 @@ export const getFactsYearly: TSelector<Record<string, TStats>> = createSelector(
         addToGroup(stats.byCurrency, codeMap[tr.outcomeInstrument], tr)
       }
 
-      addToGroup(stats.byTag, tr.tag ? tr.tag[0] : 'null', tr)
+      addToGroup(stats.byTag, tr.tag?.[0] ?? nullTagId, tr)
       addToGroup(stats.byMonth, parseDate(tr.date).getMonth().toString(), tr)
       addToGroup(stats.byWeekday, parseDate(tr.date).getDay().toString(), tr)
     }
diff --git a/src/3-widgets/transaction/TransactionList/TopBar/BulkEditModal.tsx b/src/3-widgets/transaction/TransactionList/TopBar/BulkEditModal.tsx
index 38bcb2682..dc809f348 100644
--- a/src/3-widgets/transaction/TransactionList/TopBar/BulkEditModal.tsx
+++ b/src/3-widgets/transaction/TransactionList/TopBar/BulkEditModal.tsx
@@ -15,6 +15,7 @@ import {
 } from '@mui/material'
 import { useAppDispatch } from 'store'
 import { trModel } from '5-entities/transaction'
+import { mixedTagId } from '5-entities/tag'
 import { TagList } from '5-entities/tag/ui/TagList'
 
 type BulkEditModalProps = Modify<DialogProps, { onClose: () => void }> & {
@@ -38,7 +39,7 @@ export const BulkEditModal: FC<BulkEditModalProps> = ({
   const sameComments = isSameComments(transactions)
   const types = getTypes(transactions)
   const tagType = types.income ? (types.outcome ? null : 'income') : 'outcome'
-  const commonTags = sameTags ? transactions[0]?.tag || [] : ['mixed']
+  const commonTags = sameTags ? transactions[0]?.tag || [] : [mixedTagId]
 
   const [tags, setTags] = useState(commonTags)
   const [comment, setComment] = useState(
diff --git a/src/3-widgets/transaction/TransactionList/Transaction/Transaction.Components.tsx b/src/3-widgets/transaction/TransactionList/Transaction/Transaction.Components.tsx
index 4eb8f1208..82049636b 100644
--- a/src/3-widgets/transaction/TransactionList/Transaction/Transaction.Components.tsx
+++ b/src/3-widgets/transaction/TransactionList/Transaction/Transaction.Components.tsx
@@ -8,7 +8,7 @@ import { Tooltip } from '6-shared/ui/Tooltip'
 import { useAppSelector } from 'store'
 import { accountModel } from '5-entities/account'
 import { TrType, trModel } from '5-entities/transaction'
-import { TTagPopulated, tagModel } from '5-entities/tag'
+import { TTagPopulated, nullTagId, tagModel } from '5-entities/tag'
 import { merchantModel } from '5-entities/merchant'
 import { SmartAmount } from '3-widgets/Amount'
 
@@ -36,8 +36,9 @@ export const Symbol: FC<SymbolProps> = ({
   ...rest
 }) => {
   const tags = tagModel.usePopulatedTags()
-  const mainTagId = tr.tag?.length ? tr.tag[0] : 'null'
-  const tag = tags[mainTagId]
+  const mainTagId = tr.tag?.length ? tr.tag[0] : nullTagId
+  // Fall back to the null tag: an unknown id would crash the whole list
+  const tag = tags[mainTagId] ?? tags[nullTagId]
   const { symbol, color } = getSymAndColor(trType, tag)
   return (
     <SymbolWrapper>
diff --git a/src/4-features/export/exportCSV/exportCSV.ts b/src/4-features/export/exportCSV/exportCSV.ts
index b7c320dcf..d6c64c248 100644
--- a/src/4-features/export/exportCSV/exportCSV.ts
+++ b/src/4-features/export/exportCSV/exportCSV.ts
@@ -125,7 +125,7 @@ const transactionToRowObj = (t: PopulatedTransaction): RowObj =>
     Дата: t.date,
     Создана: formatDate(t.created, 'yyyy-MM-dd HH:mm'),
     Тип: types[t.type as TrType],
-    Категория: t.tag ? t.tag[0].title : '',
+    Категория: t.tag?.[0]?.title || '',
     'Доп категории': '',
     'Со счёта': t.outcomeAccount ? t.outcomeAccount.title : '',
     Расход: !!t.outcome ? t.outcome : '',
diff --git a/src/5-entities/tag/model/constants.ts b/src/5-entities/tag/model/constants.ts
new file mode 100644
index 000000000..cf8408ce1
--- /dev/null
+++ b/src/5-entities/tag/model/constants.ts
@@ -0,0 +1,10 @@
+import type { TTagId } from '6-shared/types'
+
+/** Id of the virtual tag for transactions without a category */
+export const nullTagId: TTagId = 'null'
+
+/**
+ * Placeholder used while editing several transactions at once. It stands for
+ * "keep the tags each transaction already has" and must never be saved.
+ */
+export const mixedTagId: TTagId = 'mixed'
diff --git a/src/5-entities/tag/model/index.ts b/src/5-entities/tag/model/index.ts
index 6b6b3ea1c..196423caf 100644
--- a/src/5-entities/tag/model/index.ts
+++ b/src/5-entities/tag/model/index.ts
@@ -3,6 +3,7 @@ import { getPopulatedTags, getTags, getTagsTree } from './model'
 import { makeTag } from './makeTag'
 import { createTag, patchTag } from './thunks'
 
+export { mixedTagId, nullTagId } from './constants'
 export type { TagTreeNode } from './model'
 export type { TTagPopulated } from './populateTags'
 export type { TTagDraft } from './thunks'
diff --git a/src/5-entities/tag/model/makeTag.ts b/src/5-entities/tag/model/makeTag.ts
index 930e8923a..1b7f4aa4c 100644
--- a/src/5-entities/tag/model/makeTag.ts
+++ b/src/5-entities/tag/model/makeTag.ts
@@ -1,6 +1,7 @@
 import { v1 as uuidv1 } from 'uuid'
 import { OptionalExceptFor, TTag } from '6-shared/types'
 import { t } from 'i18next'
+import { nullTagId } from './constants'
 
 type TagDraft = OptionalExceptFor<TTag, 'user' | 'title'>
 
@@ -27,7 +28,7 @@ export const nullTag = makeTag({
   // TODO: ??? i18n
   title: t('common:tagNull'),
   user: 0,
-  id: 'null',
+  id: nullTagId,
   budgetIncome: true,
   budgetOutcome: true,
 })
diff --git a/src/5-entities/tag/ui/TagChip.tsx b/src/5-entities/tag/ui/TagChip.tsx
index c955bc75c..f2256a598 100644
--- a/src/5-entities/tag/ui/TagChip.tsx
+++ b/src/5-entities/tag/ui/TagChip.tsx
@@ -6,12 +6,12 @@ import { useTranslation } from 'react-i18next'
 import { Chip, ChipProps } from '@mui/material'
 import { CloseIcon } from '6-shared/ui/Icons'
 import { TagIcon } from '../../../6-shared/ui/TagIcon'
-import { tagModel, TTagPopulated } from '../model'
+import { mixedTagId, tagModel, TTagPopulated } from '../model'
 
 export const TagChip: FC<ChipProps & { id: TTagId }> = ({ id, ...rest }) => {
   const { t } = useTranslation()
   let tag = tagModel.usePopulatedTags()[id]
-  const label = id === 'mixed' ? t('mixedCategories') : getTagLabel(tag)
+  const label = id === mixedTagId ? t('mixedCategories') : getTagLabel(tag)
   return <Chip deleteIcon={<CloseIcon />} label={label} {...rest} />
 }
 
diff --git a/src/5-entities/tag/ui/TagSelect2.test.tsx b/src/5-entities/tag/ui/TagSelect2.test.tsx
index 98b2df3ff..c95d4fa46 100644
--- a/src/5-entities/tag/ui/TagSelect2.test.tsx
+++ b/src/5-entities/tag/ui/TagSelect2.test.tsx
@@ -9,9 +9,18 @@ import dataReducer from 'store/data'
 import { afterEach, beforeEach, describe, expect, test, vi } from 'vitest'
 import { TagSelect2 } from './TagSelect2'
 
-vi.mock('i18next', () => ({
-  t: (key: string) => key,
-}))
+vi.mock('i18next', () => {
+  const t = (key: string) => key
+  const instance = {
+    t,
+    use: () => instance,
+    init: () => Promise.resolve(t),
+    on: () => {},
+    language: 'en',
+    resolvedLanguage: 'en',
+  }
+  return { t, default: instance }
+})
 
 function makeOutcomeTag(id: string, title: string): TTag {
   return {
diff --git a/src/5-entities/transaction/helpers.ts b/src/5-entities/transaction/helpers.ts
index 8771399ea..2e3d5f2ed 100644
--- a/src/5-entities/transaction/helpers.ts
+++ b/src/5-entities/transaction/helpers.ts
@@ -37,8 +37,7 @@ export function getTime(tr: TTransaction) {
 }
 
 export function getMainTag(tr: TTransaction) {
-  if (tr.tag) return tr.tag[0]
-  else return null
+  return tr.tag?.[0] ?? null
 }
 
 export function isViewed(tr: TTransaction) {
diff --git a/src/5-entities/transaction/thunks.test.ts b/src/5-entities/transaction/thunks.test.ts
new file mode 100644
index 000000000..ecb049adf
--- /dev/null
+++ b/src/5-entities/transaction/thunks.test.ts
@@ -0,0 +1,64 @@
+import type { TTagId } from '6-shared/types'
+import { describe, expect, test } from 'vitest'
+import { store } from 'store'
+import { applyClientPatch } from 'store/data'
+import { mixedTagId } from '5-entities/tag'
+import { getMainTag } from './helpers'
+import { makeTransaction } from './makeTransaction'
+import { getTransactionsById } from './model'
+import { bulkEditTransactions } from './thunks'
+
+function makeTr(id: string, tag: TTagId[] | null) {
+  return makeTransaction({
+    id,
+    user: 0,
+    date: '2024-01-01',
+    incomeInstrument: 2,
+    incomeAccount: 'acc',
+    outcomeInstrument: 2,
+    outcomeAccount: 'acc',
+    outcome: 100,
+    tag,
+  })
+}
+
+describe('bulkEditTransactions', () => {
+  test('adds a tag keeping own tags of every transaction', () => {
+    store.dispatch(
+      applyClientPatch({
+        transaction: [makeTr('tr1', ['tag1']), makeTr('tr2', ['tag2'])],
+      })
+    )
+    store.dispatch(
+      bulkEditTransactions(['tr1', 'tr2'], { tags: [mixedTagId, 'tag3'] })
+    )
+    const transactions = getTransactionsById(store.getState())
+    expect(transactions.tr1.tag).toEqual(['tag1', 'tag3'])
+    expect(transactions.tr2.tag).toEqual(['tag2', 'tag3'])
+  })
+
+  test('never writes the "mixed" placeholder to a transaction without tags', () => {
+    store.dispatch(
+      applyClientPatch({
+        transaction: [makeTr('tr3', ['tag1']), makeTr('tr4', null)],
+      })
+    )
+    store.dispatch(
+      bulkEditTransactions(['tr3', 'tr4'], { tags: [mixedTagId, 'tag3'] })
+    )
+    const transactions = getTransactionsById(store.getState())
+    expect(transactions.tr3.tag).toEqual(['tag1', 'tag3'])
+    expect(transactions.tr4.tag).toEqual(['tag3'])
+  })
+})
+
+describe('getMainTag', () => {
+  test('returns null for transactions without tags', () => {
+    expect(getMainTag(makeTr('tr5', null))).toBe(null)
+    expect(getMainTag(makeTr('tr6', []))).toBe(null)
+  })
+
+  test('returns the first tag', () => {
+    expect(getMainTag(makeTr('tr7', ['tag1', 'tag2']))).toBe('tag1')
+  })
+})
diff --git a/src/5-entities/transaction/thunks.ts b/src/5-entities/transaction/thunks.ts
index a7d34ef1a..99963e414 100644
--- a/src/5-entities/transaction/thunks.ts
+++ b/src/5-entities/transaction/thunks.ts
@@ -8,6 +8,7 @@ import {
   TTransactionId,
 } from '6-shared/types'
 import { applyClientPatch } from 'store/data'
+import { mixedTagId, nullTagId } from '5-entities/tag'
 import { getTransactionsById } from './model'
 import { isViewed } from './helpers'
 
@@ -136,9 +137,10 @@ const modifyTags = (prevTags: string[] | null, newTags?: string[]) => {
   if (!newTags) return prevTags
   let result: TTagId[] = []
   const addId = (id: string) =>
-    result.includes(id) || id === 'null' ? '' : result.push(id)
+    result.includes(id) || id === nullTagId ? '' : result.push(id)
   newTags?.forEach(id => {
-    if (id === 'mixed' && prevTags) prevTags.forEach(addId)
+    // mixedTagId is a placeholder for the transaction's own tags, never a real id
+    if (id === mixedTagId) prevTags?.forEach(addId)
     else addId(id)
   })
   return result