diff --git a/app/controllers/customercontroller.js b/app/controllers/customercontroller.js
index 37e32e6..84fa0be 100644
--- a/app/controllers/customercontroller.js
+++ b/app/controllers/customercontroller.js
@@ -539,6 +539,23 @@ exports.search = async (req, res) => {
 async function findAndRespond(customerId, res) {
     try {
         const customer = await Customer.findByPk(customerId);
+        if (!customer) {
+            // The master-key branch in getCustomerById calls
+            // findAndRespond without first verifying the row exists,
+            // so a non-existent customerId used to land here and
+            // return `200 { customers: null }` — a confusing
+            // success envelope wrapping an empty body. The scoped
+            // (non-master) branch already short-circuits to 403 on
+            // `GetCustomerCompanyId(id) === -1`, so it never hit this
+            // fallthrough. Make the master path return a proper 404
+            // so both paths agree that "no such customer id" is
+            // a not-found, not a success.
+            //
+            // The `defaultScope` on the Customer model filters archived
+            // rows, so findByPk also returns null for soft-deleted
+            // customers — soft-deletes correctly surface as 404 too.
+            return res.status(404).json({ message: "Not found." });
+        }
         return res.status(200).json({
             message: "Successfully retrieved the customer with CustomerId " + customerId,
             customers: customer,
diff --git a/tests/api/customer.test.js b/tests/api/customer.test.js
index f417886..819679c 100644
--- a/tests/api/customer.test.js
+++ b/tests/api/customer.test.js
@@ -101,4 +101,39 @@ describe('GET /v1/customer/:id', () => {
         // 500 would indicate the error escaped its try/catch.
         expect(res.status).toBe(403);
     });
+
+    test('master key + non-existent customer id returns 404, not 200+null', async () => {
+        // Drive the master-key branch of getCustomerById directly so we
+        // can pin findAndRespond's null-row path. Before the fix, a
+        // null-returning findByPk emitted `200 { customers: null }` —
+        // a success envelope wrapping an empty body that any client
+        // checking `body.customers.custId` would crash on.
+        //
+        // Use the P5-M _setDbForTesting seam on auth.js: stub
+        // ApiMaster.findOne to return a row so real `IsMaster(authKey)`
+        // resolves true. vi.spyOn(auth.isMaster) doesn't intercept the
+        // captured `const IsMaster = auth.isMaster` reference in the
+        // controller — the seam is the documented hook.
+        const auth = require('../../app/middleware/auth.js');
+        const customer = require('../../app/controllers/customercontroller.js');
+        auth._setDbForTesting({
+            ApiMaster: { findOne: vi.fn().mockResolvedValue({ amId: 1 }) },
+            ApiKey: { findOne: vi.fn() },
+        });
+        try {
+            const db = require('../../app/config/db.config.js');
+            db.Customer.findByPk = vi.fn().mockResolvedValue(null);
+            const req = { get: (h) => (h === 'authKey' ? 'master' : undefined), params: { id: 999999 } };
+            let captured = null;
+            const res = {
+                status(code) { this._code = code; return this; },
+                json(body) { captured = { code: this._code, body }; return this; },
+            };
+            await customer.getCustomerById(req, res);
+            expect(captured.code).toBe(404);
+            expect(captured.body.message).toMatch(/not found/i);
+        } finally {
+            auth._setDbForTesting(null);
+        }
+    });
 });