diff --git a/packages/runtime-core/src/apiApp.ts b/packages/runtime-core/src/apiApp.ts
index c526e84f..71b75821 100644
--- a/packages/runtime-core/src/apiApp.ts
+++ b/packages/runtime-core/src/apiApp.ts
@@ -4,7 +4,7 @@ import { ComponentPublicInstance } from './componentProxy'
 import { Directive, validateDirectiveName } from './directives'
 import { RootRenderFunction } from './renderer'
 import { InjectionKey } from './apiInject'
-import { isFunction, NO } from '@vue/shared'
+import { isFunction, NO, isObject } from '@vue/shared'
 import { warn } from './warning'
 import { createVNode } from './vnode'
 
@@ -163,9 +163,14 @@ export function createAppAPI<HostNode, HostElement>(
       mount(
         rootComponent: Component,
         rootContainer: HostElement,
-        rootProps?: Data
+        rootProps?: Data | null
       ): any {
         if (!isMounted) {
+          if (!isObject(rootProps)) {
+            __DEV__ &&
+              warn(`root props passed to app.mount() must be an object.`)
+            rootProps = null
+          }
           const vnode = createVNode(rootComponent, rootProps)
           // store app context on the root VNode.
           // this will be set on the root instance on initial mount.