diff --git a/app/api/invoices/[id]/route.ts b/app/api/invoices/[id]/route.ts
index ec8f65c8..936b1bef 100644
--- a/app/api/invoices/[id]/route.ts
+++ b/app/api/invoices/[id]/route.ts
@@ -148,7 +148,7 @@ export const PATCH = withRouteContext<{ params: Promise<{ id: string }> }>(
     // received self-billing document) may be edited.
     const { data: existing, error: fetchError } = await supabase
       .from('invoices')
-      .select('id, status, invoice_number, journal_entry_id, is_self_billed, credited_invoice_id')
+      .select('id, status, invoice_number, journal_entry_id, is_self_billed, credited_invoice_id, deduction_personnummer_encrypted, deduction_personnummer_last4')
       .eq('id', id)
       .eq('company_id', companyId!)
       .single()
@@ -186,6 +186,15 @@ export const PATCH = withRouteContext<{ params: Promise<{ id: string }> }>(
       customer,
       documentType,
       input,
+      // The stored personnummer exists only as ciphertext (client sees _last4
+      // at most), so an edit that leaves the field empty keeps it rather than
+      // failing ROT/RUT validation (issue #1175).
+      existingPersonnummer: existing.deduction_personnummer_encrypted
+        ? {
+            encrypted: existing.deduction_personnummer_encrypted,
+            last4: existing.deduction_personnummer_last4 ?? null,
+          }
+        : null,
     })
     if (!build.ok) {
       if ('dbError' in build) {
diff --git a/components/invoices/InvoiceEditor.tsx b/components/invoices/InvoiceEditor.tsx
index dd73c588..9cb7337c 100644
--- a/components/invoices/InvoiceEditor.tsx
+++ b/components/invoices/InvoiceEditor.tsx
@@ -2074,7 +2074,11 @@ export default function InvoiceEditor(props: InvoiceEditorProps = { mode: 'creat
                       {...register('deduction_personnummer')}
                     />
                     <p className="text-xs text-muted-foreground">
-                      {t('deduction_personnummer_hint')}
+                      {/* Stored pn exists only as ciphertext: an empty field on
+                          edit keeps it server-side instead of failing validation. */}
+                      {initial?.deduction_personnummer_last4
+                        ? t('deduction_personnummer_kept_hint', { last4: initial.deduction_personnummer_last4 })
+                        : t('deduction_personnummer_hint')}
                     </p>
                   </div>
                   {hasAnyRotLine && (
diff --git a/lib/invoices/__tests__/build-invoice-write.test.ts b/lib/invoices/__tests__/build-invoice-write.test.ts
index a6bf9eff..9dbd5e38 100644
--- a/lib/invoices/__tests__/build-invoice-write.test.ts
+++ b/lib/invoices/__tests__/build-invoice-write.test.ts
@@ -159,3 +159,73 @@ describe('buildInvoiceWriteData', () => {
     expect(result.items[0]).toMatchObject({ line_type: 'text', line_total: 0, vat_amount: 0 })
   })
 })
+
+describe('buildInvoiceWriteData stored ROT/RUT personnummer (edit path)', () => {
+  const rutItem = {
+    description: 'Städning',
+    quantity: 10,
+    unit: 'tim',
+    unit_price: 500,
+    vat_rate: 25,
+    deduction_type: 'rut' as const,
+    labor_hours: 10,
+  }
+
+  it('keeps the stored ciphertext when the edit leaves personnummer empty', async () => {
+    const { supabase, enqueue } = createQueuedMockSupabase()
+    enqueue({ data: { vat_registered: true }, error: null })
+
+    const customer = makeCustomer({ customer_type: 'individual' })
+    const result = await buildInvoiceWriteData({
+      supabase: supabase as unknown as SupabaseClient,
+      companyId: 'company-1',
+      customer,
+      documentType: 'invoice',
+      input: { ...baseHeader, items: [rutItem] },
+      existingPersonnummer: { encrypted: 'stored-ciphertext', last4: '1234' },
+    })
+
+    expect(result.ok).toBe(true)
+    if (!result.ok) return
+    expect(result.invoiceFields.deduction_personnummer_encrypted).toBe('stored-ciphertext')
+    expect(result.invoiceFields.deduction_personnummer_last4).toBe('1234')
+  })
+
+  it('still rejects a deduction invoice with no personnummer anywhere (create path)', async () => {
+    const { supabase, enqueue } = createQueuedMockSupabase()
+    enqueue({ data: { vat_registered: true }, error: null })
+
+    const customer = makeCustomer({ customer_type: 'individual' })
+    const result = await buildInvoiceWriteData({
+      supabase: supabase as unknown as SupabaseClient,
+      companyId: 'company-1',
+      customer,
+      documentType: 'invoice',
+      input: { ...baseHeader, items: [rutItem] },
+    })
+
+    expect(result.ok).toBe(false)
+    if (result.ok) return
+    expect('code' in result && result.code).toBe('INVOICE_CREATE_ROT_RUT_VALIDATION')
+  })
+
+  it('does not resurrect the stored personnummer when all deduction lines are removed', async () => {
+    const { supabase, enqueue } = createQueuedMockSupabase()
+    enqueue({ data: { vat_registered: true }, error: null })
+
+    const customer = makeCustomer({ customer_type: 'individual' })
+    const result = await buildInvoiceWriteData({
+      supabase: supabase as unknown as SupabaseClient,
+      companyId: 'company-1',
+      customer,
+      documentType: 'invoice',
+      input: { ...baseHeader, items: [{ description: 'Vanlig tjänst', quantity: 1, unit: 'st', unit_price: 100, vat_rate: 25 }] },
+      existingPersonnummer: { encrypted: 'stored-ciphertext', last4: '1234' },
+    })
+
+    expect(result.ok).toBe(true)
+    if (!result.ok) return
+    expect(result.invoiceFields.deduction_personnummer_encrypted).toBeNull()
+    expect(result.invoiceFields.deduction_personnummer_last4).toBeNull()
+  })
+})
diff --git a/lib/invoices/build-invoice-write.ts b/lib/invoices/build-invoice-write.ts
index 07b0c977..6e048af2 100644
--- a/lib/invoices/build-invoice-write.ts
+++ b/lib/invoices/build-invoice-write.ts
@@ -158,8 +158,15 @@ export async function buildInvoiceWriteData(params: {
   customer: Customer
   documentType: InvoiceDocumentType
   input: InvoiceWriteInput
+  /**
+   * Update path only: the stored encrypted personnummer of the draft being
+   * edited. The plaintext is never rehydratable client-side (only _last4 is),
+   * so an edit that leaves the field empty keeps these stored values instead
+   * of failing ROT/RUT validation or wiping the ciphertext.
+   */
+  existingPersonnummer?: { encrypted: string; last4: string | null } | null
 }): Promise<BuildInvoiceWriteResult> {
-  const { supabase, companyId, customer, documentType, input } = params
+  const { supabase, companyId, customer, documentType, input, existingPersonnummer } = params
   const items = input.items
 
   const vatRules = getVatRules(customer.customer_type, customer.vat_number_validated)
@@ -308,7 +315,6 @@ export async function buildInvoiceWriteData(params: {
     }
     const housingProvided = fastighetProvided || (apartmentProvided && brfProvided)
     const personnummerRaw = input.deduction_personnummer?.trim() || ''
-    const personnummerProvided = personnummerRaw.length > 0
 
     const validateInput = items.map((item) => ({
       unit_price: item.unit_price,
@@ -317,6 +323,14 @@ export async function buildInvoiceWriteData(params: {
       labor_hours: item.labor_hours ?? null,
       housing_designation: item.housing_designation ?? null,
     }))
+
+    // Editing a draft: the stored personnummer only exists as ciphertext, so
+    // the client cannot resend it. An empty field on an invoice that still has
+    // deduction lines means "keep the stored one", not "remove it".
+    const hasDeductionItems = validateInput.some((item) => item.deduction_type != null)
+    const keepStoredPersonnummer =
+      personnummerRaw.length === 0 && hasDeductionItems && !!existingPersonnummer
+    const personnummerProvided = personnummerRaw.length > 0 || keepStoredPersonnummer
     const validation = validateRotRut(validateInput, personnummerProvided, housingProvided)
     if (validation.errors.length > 0) {
       return {
@@ -330,7 +344,10 @@ export async function buildInvoiceWriteData(params: {
     // never touches the DB: only the AES-256-GCM ciphertext + the last four
     // digits go into invoices columns.
     deductionTotal = computeInvoiceDeductionTotal(validateInput)
-    if (personnummerProvided) {
+    if (keepStoredPersonnummer && existingPersonnummer) {
+      deductionPersonnummerEncrypted = existingPersonnummer.encrypted
+      deductionPersonnummerLast4 = existingPersonnummer.last4
+    } else if (personnummerProvided) {
       const pnValid = validatePersonnummer(personnummerRaw)
       if (!pnValid.valid) {
         return { ok: false, code: 'INVOICE_CREATE_ROT_RUT_PERSONNUMMER_INVALID', details: { error: pnValid.error } }
diff --git a/messages/en.json b/messages/en.json
index 4f05a42c..1bfbd344 100644
--- a/messages/en.json
+++ b/messages/en.json
@@ -2914,6 +2914,7 @@
     "deduction_personnummer_label": "Personal identity number (personnummer)",
     "deduction_personnummer_placeholder": "YYYYMMDD-NNNN",
     "deduction_personnummer_hint": "Encrypted before storage. Only the last four digits are shown on the invoice.",
+    "deduction_personnummer_kept_hint": "The saved personal number (****{last4}) is kept if the field is left empty. Enter one only to replace it.",
     "deduction_housing_label": "Property designation (fastighetsbeteckning)",
     "deduction_housing_placeholder": "e.g. Stockholm Vasastan 1:23",
     "deduction_housing_hint": "Required for ROT deductions (not needed for RUT).",
diff --git a/messages/sv.json b/messages/sv.json
index d76c88c3..ac7cd8b8 100644
--- a/messages/sv.json
+++ b/messages/sv.json
@@ -2914,6 +2914,7 @@
     "deduction_personnummer_label": "Personnummer",
     "deduction_personnummer_placeholder": "ÅÅÅÅMMDD-NNNN",
     "deduction_personnummer_hint": "Krypteras innan lagring. Endast de fyra sista siffrorna visas på fakturan.",
+    "deduction_personnummer_kept_hint": "Sparat personnummer (****{last4}) behålls om fältet lämnas tomt. Fyll i endast för att byta.",
     "deduction_housing_label": "Fastighetsbeteckning",
     "deduction_housing_placeholder": "t.ex. Stockholm Vasastan 1:23",
     "deduction_housing_hint": "Krävs för ROT-avdrag (RUT behöver inte detta fält).",