diff --git a/src/apis/inject.ts b/src/apis/inject.ts
index 44183798..75b53013 100644
--- a/src/apis/inject.ts
+++ b/src/apis/inject.ts
@@ -48,10 +48,6 @@ export function inject(
   defaultValue?: unknown,
   treatDefaultAsFactory = false
 ) {
-  if (!key) {
-    return defaultValue
-  }
-
   const vm = getCurrentInstance()?.proxy
   if (!vm) {
     __DEV__ &&
@@ -59,6 +55,11 @@ export function inject(
     return
   }
 
+  if (!key) {
+    __DEV__ && warn(`injection "${String(key)}" not found.`, vm)
+    return defaultValue
+  }
+
   const val = resolveInject(key, vm)
   if (val !== NOT_FOUND) {
     return val
diff --git a/test/v3/runtime-core/apiInject.spec.ts b/test/v3/runtime-core/apiInject.spec.ts
index f4593eae..1ed7d5cb 100644
--- a/test/v3/runtime-core/apiInject.spec.ts
+++ b/test/v3/runtime-core/apiInject.spec.ts
@@ -242,6 +242,28 @@ describe('api: provide/inject', () => {
     expect(`[Vue warn]: Injection "foo" not found`).toHaveBeenWarned()
   })
 
+  it('should warn unfound w/ injectionKey is undefined', () => {
+    const Provider = {
+      setup() {
+        return () => h(Consumer)
+      },
+    }
+
+    const Consumer = {
+      setup() {
+        // The emulation does not use TypeScript
+        const foo = inject(undefined as unknown as string)
+        expect(foo).toBeUndefined()
+        return () => h('div', foo as unknown as string)
+      },
+    }
+
+    const root = document.createElement('div')
+    const vm = createApp(Provider).mount(root)
+    expect(vm.$el.outerHTML).toBe(`<div></div>`)
+    expect(`[Vue warn]: injection "undefined" not found.`).toHaveBeenWarned()
+  })
+
   it('should not self-inject', () => {
     const Comp = {
       setup() {