diff --git a/app/schemas/invoicejob.schema.js b/app/schemas/invoicejob.schema.js
index 6c4a399..c11bdb5 100644
--- a/app/schemas/invoicejob.schema.js
+++ b/app/schemas/invoicejob.schema.js
@@ -8,16 +8,28 @@ const intIdParam = z.object({
     id: z.coerce.number().int().positive(),
 });
 
+// `injbAmount` is the per-line monetary value on an invoice. zod's
+// `.number()` rejects NaN by default but allows Infinity / -Infinity
+// through, and the column is a Sequelize DOUBLE — `inf` lands in the
+// database fine and contaminates any consumer doing arithmetic
+// (invoice totals, aging buckets, CSV exports). Pin to finite real
+// numbers at the boundary. Zero and negative values still pass (a
+// $0 reference line and a credit/discount line are both real
+// accounting uses).
+const injbAmountField = z.coerce.number().finite({
+    message: 'injbAmount must be a finite number.',
+});
+
 const createInvoiceJobBody = z.object({
     injbInvId: z.coerce.number().int().positive(),
     injbJobId: z.coerce.number().int().positive(),
-    injbAmount: z.coerce.number(),
+    injbAmount: injbAmountField,
 }).strict({
     message: 'Unexpected field in body. Whitelist: injbInvId, injbJobId, injbAmount.',
 });
 
 const updateInvoiceJobBody = z.object({
-    injbAmount: z.coerce.number().optional(),
+    injbAmount: injbAmountField.optional(),
 }).strict({
     message: 'Unexpected field in body. Whitelist: injbAmount.',
 });
diff --git a/tests/api/invoicejob.test.js b/tests/api/invoicejob.test.js
index ee90a96..623db8c 100644
--- a/tests/api/invoicejob.test.js
+++ b/tests/api/invoicejob.test.js
@@ -54,4 +54,36 @@ describe('InvoiceJob body validation', () => {
         const res = await request(app).post('/v1/invoicejob').set('authKey', 'any').send({ injbInvId: 1, injbJobId: 1, injbAmount: 1, bogus: 'no' });
         expect(res.status).toBe(400);
     });
+
+    test('POST accepts negative injbAmount (credit / discount lines)', async () => {
+        const res = await request(app).post('/v1/invoicejob').set('authKey', 'any')
+            .send({ injbInvId: 1, injbJobId: 1, injbAmount: -50 });
+        expect(res.status).not.toBe(400);
+    });
+
+    test('POST accepts zero injbAmount (reference / informational lines)', async () => {
+        // Unlike cpayAmount (which rejects 0 because a $0 payment is
+        // operator error), invoice lines at $0 are a real accounting
+        // case: a courtesy line, a placeholder, or a service-rendered
+        // reference. Pin this so a future tightening surfaces here.
+        const res = await request(app).post('/v1/invoicejob').set('authKey', 'any')
+            .send({ injbInvId: 1, injbJobId: 1, injbAmount: 0 });
+        expect(res.status).not.toBe(400);
+    });
+
+    test('PATCH rejects non-finite injbAmount (JSON `null` is the only way to send Infinity through JSON, but a string \\"Infinity\\" hits coerce-to-number)', async () => {
+        // JSON has no literal for Infinity, but `z.coerce.number()`
+        // happily turns the string "Infinity" into the float — and
+        // Sequelize's DOUBLE column will store it as `inf`. The
+        // .finite() refinement catches it at the schema boundary.
+        const res = await request(app).patch('/v1/invoicejob/1').set('authKey', 'any')
+            .send({ injbAmount: 'Infinity' });
+        expect(res.status).toBe(400);
+    });
+
+    test('PATCH rejects -Infinity injbAmount via the same coerce path', async () => {
+        const res = await request(app).patch('/v1/invoicejob/1').set('authKey', 'any')
+            .send({ injbAmount: '-Infinity' });
+        expect(res.status).toBe(400);
+    });
 });