@@ -1073,6 +1073,31 @@ describe('mergeConfig', () => {
     expect(config.server.ws?.overlay).toBeUndefined()
   })
 
+  test('`mergeConfig` does not crash when `server.ws` is false and `server.hmr` is merged', () => {
+    const baseConfig = defineConfig({
+      server: {
+        ws: false,
+        hmr: {
+          host: 'localhost',
+        },
+      },
+    })
+
+    const newConfig = defineConfig({
+      server: {
+        hmr: {
+          port: 5173,
+        },
+      },
+    })
+
+    const mergedConfig = mergeConfig(baseConfig, newConfig)
+
+    expect(mergedConfig.server.ws).toBe(false)
+    expect(mergedConfig.server.hmr).toBeTypeOf('object')
+    expect(mergedConfig.server.hmr).toBeTruthy()
+  })
+
   test('resolveConfig properly syncs hmr and ws', async () => {
     const config = await resolveConfig(
       {
 
@@ -1479,11 +1479,10 @@ function mergeConfigRecursively(
   }
   if (rootPath === 'server.hmr') {
     for (const key of wsOptionKeys) {
-      Object.defineProperty(
-        merged,
-        key,
-        Object.getOwnPropertyDescriptor(defaults, key)!,
-      )
+      const descriptor = Object.getOwnPropertyDescriptor(defaults, key)
+      if (descriptor) {
+        Object.defineProperty(merged, key, descriptor)
+      }
     }
   }
Original file line number	Diff line number	Diff line change
`@@ -1479,11 +1479,10 @@ function mergeConfigRecursively(`
`1479`	`1479`	`}`
`1480`	`1480`	`if (rootPath === 'server.hmr') {`
`1481`	`1481`	`for (const key of wsOptionKeys) {`
`1482`		`- Object.defineProperty(`
`1483`		`- merged,`
`1484`		`- key,`
`1485`		`- Object.getOwnPropertyDescriptor(defaults, key)!,`
`1486`		`- )`
	`1482`	`+ const descriptor = Object.getOwnPropertyDescriptor(defaults, key)`
	`1483`	`+ if (descriptor) {`
	`1484`	`+ Object.defineProperty(merged, key, descriptor)`
	`1485`	`+ }`
`1487`	`1486`	`}`
`1488`	`1487`	`}`
`1489`	`1488`