diff --git a/src/createInstance.ts b/src/createInstance.ts
index e37c8c2ad..9e2067cbe 100644
--- a/src/createInstance.ts
+++ b/src/createInstance.ts
@@ -246,7 +246,18 @@ export function createInstance(
           for (const [k, v] of Object.entries(
             global.mocks as { [key: string]: any }
           )) {
-            ;(this as any)[k] = v
+            // the proxy rejects `$x` when `x` is an internal instance field
+            // (e.g. `$u` from uView), so we define these directly on the context
+            if (k[0] === '$' && k.slice(1) in this.$) {
+              Object.defineProperty((this.$ as any).ctx, k, {
+                configurable: true,
+                enumerable: true,
+                writable: true,
+                value: v
+              })
+            } else {
+              ;(this as any)[k] = v
+            }
           }
         }
       }
diff --git a/tests/mountingOptions/mocks.spec.ts b/tests/mountingOptions/mocks.spec.ts
index 39909932c..e1a8f3fe5 100644
--- a/tests/mountingOptions/mocks.spec.ts
+++ b/tests/mountingOptions/mocks.spec.ts
@@ -151,4 +151,43 @@ describe('mocks', () => {
     expect(wrapper.text()).toContain('hello')
     expect(wrapper.text()).toContain('mocked')
   })
+
+  it('mocks a global property named after an internal instance field', () => {
+    // `$u` (uView) matches the internal `u` field of the component instance
+    const Foo = defineComponent({
+      template: '<div>{{ $u.title }} {{ title }}</div>',
+      data() {
+        return { title: (this as any).$u.title }
+      }
+    })
+
+    const wrapper = mount(Foo, {
+      global: {
+        mocks: { $u: { title: 'mocked' } }
+      }
+    })
+
+    expect(wrapper.text()).toBe('mocked mocked')
+  })
+
+  it('overrides a global property named after an internal instance field', () => {
+    const Foo = defineComponent({
+      template: '<div>{{ $u.title }}</div>'
+    })
+
+    const wrapper = mount(Foo, {
+      global: {
+        plugins: [
+          {
+            install(app) {
+              app.config.globalProperties.$u = { title: 'real' }
+            }
+          }
+        ],
+        mocks: { $u: { title: 'mocked' } }
+      }
+    })
+
+    expect(wrapper.text()).toBe('mocked')
+  })
 })
diff --git a/tests/renderToString.spec.ts b/tests/renderToString.spec.ts
index 5e515b37b..4fc087661 100644
--- a/tests/renderToString.spec.ts
+++ b/tests/renderToString.spec.ts
@@ -83,4 +83,16 @@ describe('renderToString', () => {
 
     spy.mockRestore()
   })
+
+  it('applies a global mock named after an internal instance field', async () => {
+    const Component = defineComponent({
+      template: '<div>{{ $u.title }}</div>'
+    })
+
+    const contents = await renderToString(Component, {
+      global: { mocks: { $u: { title: 'mocked' } } }
+    })
+
+    expect(contents).toBe('<div>mocked</div>')
+  })
 })