diff --git a/app/schemas/billingtype.schema.js b/app/schemas/billingtype.schema.js
index 98724a1..851053f 100644
--- a/app/schemas/billingtype.schema.js
+++ b/app/schemas/billingtype.schema.js
@@ -8,9 +8,25 @@ const intIdParam = z.object({
     id: z.coerce.number().int().positive(),
 });
 
+// `btHourlyRate` is a Sequelize DOUBLE column for the hourly rate
+// charged against a BillingType. `.nonnegative()` blocks negatives
+// (a -$50/hr rate is operator error) but DOES NOT block `Infinity`
+// — `Infinity >= 0` is true. The coerce path also turns the string
+// `"Infinity"` into the float, so JSON without an Infinity literal
+// can still land `inf` in the column and contaminate downstream
+// invoice/time-entry totals.
+//
+// Chain `.finite()` ahead of `.nonnegative()` to reject the
+// infinities. Zero remains a valid rate (pro-bono / internal-only
+// billing entries). Mirrors the cpayAmount / injbAmount / polPrice
+// validators (#172 / #180 / #194).
+const btHourlyRateField = z.coerce.number()
+    .finite({ message: 'btHourlyRate must be a finite number.' })
+    .nonnegative();
+
 const createBillingTypeBody = z.object({
     btName: z.string().min(1).max(255),
-    btHourlyRate: z.coerce.number().nonnegative(),
+    btHourlyRate: btHourlyRateField,
     btCompId: z.coerce.number().int().positive().optional(),
 }).strict({
     message: 'Unexpected field in body. Whitelist: btName, btHourlyRate, btCompId.',
@@ -18,7 +34,7 @@ const createBillingTypeBody = z.object({
 
 const updateBillingTypeBody = z.object({
     btName: z.string().min(1).max(255).optional(),
-    btHourlyRate: z.coerce.number().nonnegative().optional(),
+    btHourlyRate: btHourlyRateField.optional(),
 }).strict({
     message: 'Unexpected field in body. Whitelist: btName, btHourlyRate.',
 });
diff --git a/tests/api/billingtype.test.js b/tests/api/billingtype.test.js
index 81a1040..12a9953 100644
--- a/tests/api/billingtype.test.js
+++ b/tests/api/billingtype.test.js
@@ -168,4 +168,42 @@ describe('BillingType body validation', () => {
             .send({ btName: 'Standard' });
         expect(res.status).toBe(400);
     });
+
+    test('POST rejects non-finite btHourlyRate (string "Infinity" coerces past nonnegative())', async () => {
+        // .nonnegative() allows Infinity (Infinity >= 0 is true).
+        // .finite() in the validator catches it before .nonnegative().
+        const res = await request(app)
+            .post('/v1/billingtype')
+            .set('authKey', 'any')
+            .send({ btName: 'Standard', btHourlyRate: 'Infinity' });
+        expect(res.status).toBe(400);
+    });
+
+    test('POST still rejects negative btHourlyRate', async () => {
+        // Pin the existing .nonnegative() guard so .finite() doesn't
+        // accidentally relax the negative-block when refactoring.
+        const res = await request(app)
+            .post('/v1/billingtype')
+            .set('authKey', 'any')
+            .send({ btName: 'Standard', btHourlyRate: -50 });
+        expect(res.status).toBe(400);
+    });
+
+    test('POST accepts zero btHourlyRate (pro-bono / internal billing)', async () => {
+        // Zero is a legitimate rate (pro-bono engagements, internal-only
+        // entries). .finite() + .nonnegative() should let it through.
+        const res = await request(app)
+            .post('/v1/billingtype')
+            .set('authKey', 'any')
+            .send({ btName: 'Pro-bono', btHourlyRate: 0 });
+        expect(res.status).not.toBe(400);
+    });
+
+    test('PATCH rejects non-finite btHourlyRate', async () => {
+        const res = await request(app)
+            .patch('/v1/billingtype/1')
+            .set('authKey', 'any')
+            .send({ btHourlyRate: '-Infinity' });
+        expect(res.status).toBe(400);
+    });
 });