diff --git a/components/invoices/InvoiceEditor.tsx b/components/invoices/InvoiceEditor.tsx
index fac31b17..a47635ec 100644
--- a/components/invoices/InvoiceEditor.tsx
+++ b/components/invoices/InvoiceEditor.tsx
@@ -806,6 +806,23 @@ export default function InvoiceEditor(props: InvoiceEditorProps = { mode: 'creat
     )
   }
 
+  // The form always carries the self-billing fields (they default to '' in both
+  // create and edit mode). This editor's normal create/draft/edit flows never
+  // use self-billing, that goes through the dedicated /api/invoices/self-billed
+  // path, so drop these empty carriers before spreading the form data into the
+  // /api/invoices (or PATCH) body: a bare external_invoice_number: '' otherwise
+  // trips the shared CreateInvoiceSchema's min(1). Belt-and-suspenders; the
+  // server schema also coerces '' to undefined for these fields.
+  function stripSelfBillingFields(data: FormData): FormData {
+    const {
+      external_invoice_number: _ein,
+      self_billing_agreement_ref: _sbar,
+      received_date: _rd,
+      ...rest
+    } = data
+    return rest
+  }
+
   // Self-billing path: no review dialog, no PDF, no send: it arrives already
   // booked. POST straight to the dedicated endpoint and open the verifikat.
   async function handleSelfBilledSubmit(data: FormData) {
@@ -943,7 +960,7 @@ export default function InvoiceEditor(props: InvoiceEditorProps = { mode: 'creat
       return rest
     })
     const sanitizedPayload: CreateInvoiceInput & { default_dimensions: Record<string, string> } = {
-      ...(pendingData as CreateInvoiceInput),
+      ...(stripSelfBillingFields(pendingData) as CreateInvoiceInput),
       ore_rounding: oreRounding,
       // Invoice-level default dims: always sent so an edited draft can clear
       // them; {} means "no defaults".
@@ -1020,7 +1037,7 @@ export default function InvoiceEditor(props: InvoiceEditorProps = { mode: 'creat
       return rest
     })
     const payload: CreateInvoiceInput & { default_dimensions: Record<string, string> } = {
-      ...(data as CreateInvoiceInput),
+      ...(stripSelfBillingFields(data) as CreateInvoiceInput),
       save_as_draft: true,
       ore_rounding: oreRounding,
       default_dimensions: defaultDims,
@@ -1079,7 +1096,7 @@ export default function InvoiceEditor(props: InvoiceEditorProps = { mode: 'creat
       return rest
     })
     const payload: CreateInvoiceInput & { default_dimensions: Record<string, string> } = {
-      ...(data as CreateInvoiceInput),
+      ...(stripSelfBillingFields(data) as CreateInvoiceInput),
       ore_rounding: oreRounding,
       default_dimensions: defaultDims,
       items: sanitizedItems as CreateInvoiceInput['items'],
diff --git a/lib/api/__tests__/schemas.test.ts b/lib/api/__tests__/schemas.test.ts
index 0f627eae..0fa78dcb 100644
--- a/lib/api/__tests__/schemas.test.ts
+++ b/lib/api/__tests__/schemas.test.ts
@@ -384,6 +384,37 @@ describe('CreateInvoiceSchema', () => {
     }))
     expect(result.success).toBe(true)
   })
+
+  // Regression: the dashboard invoice form always sends the self-billing fields
+  // (default '' for a normal invoice). Empty strings must read as "not
+  // provided", not fail min(1)/isoDate, or every regular invoice create 400s.
+  it('treats empty self-billing strings as omitted (not a validation error)', () => {
+    const result = CreateInvoiceSchema.safeParse(validInvoice({
+      external_invoice_number: '',
+      self_billing_agreement_ref: '',
+      received_date: '',
+    }))
+    expect(result.success).toBe(true)
+    if (result.success) {
+      expect(result.data.external_invoice_number).toBeUndefined()
+      expect(result.data.self_billing_agreement_ref).toBeUndefined()
+      expect(result.data.received_date).toBeUndefined()
+    }
+  })
+
+  it('still accepts real self-billing values', () => {
+    const result = CreateInvoiceSchema.safeParse(validInvoice({
+      is_self_billed: true,
+      external_invoice_number: 'CUST-2026-014',
+      self_billing_agreement_ref: 'AVTAL-7',
+      received_date: '2026-07-07',
+    }))
+    expect(result.success).toBe(true)
+    if (result.success) {
+      expect(result.data.external_invoice_number).toBe('CUST-2026-014')
+      expect(result.data.received_date).toBe('2026-07-07')
+    }
+  })
 })
 
 describe('UpdateInvoiceSchema', () => {
diff --git a/lib/api/schemas.ts b/lib/api/schemas.ts
index 6909694d..efd6e0ea 100644
--- a/lib/api/schemas.ts
+++ b/lib/api/schemas.ts
@@ -401,9 +401,22 @@ export const CreateInvoiceSchema = z.object({
   // invoice. A plain optional flag (no schema refine) so UpdateInvoiceSchema's
   // .omit() keeps working on this object.
   is_self_billed: z.boolean().optional(),
-  external_invoice_number: z.string().min(1).max(64).optional(),
-  self_billing_agreement_ref: z.string().max(128).optional(),
-  received_date: isoDate.optional(),
+  // The dashboard invoice form always sends these self-billing fields (default
+  // '' in create/edit mode) even for a normal invoice, so an empty string must
+  // read as "not provided", not fail validation. Otherwise a plain
+  // external_invoice_number: '' trips the min(1) and 400s every regular invoice
+  // create. Required-when-self-billed is still enforced in the v1 route via a
+  // falsy check after parse, so normalising '' -> undefined here is safe.
+  external_invoice_number: z
+    .union([z.string().min(1).max(64), z.literal('')])
+    .transform((v) => v || undefined)
+    .optional(),
+  self_billing_agreement_ref: z
+    .string()
+    .max(128)
+    .transform((v) => v || undefined)
+    .optional(),
+  received_date: optionalIsoDate,
   items: z.array(CreateInvoiceItemSchema).min(1, 'At least one item is required'),
 })