diff --git a/packages/core/src/utilities/isPlainObject.ts b/packages/core/src/utilities/isPlainObject.ts
new file mode 100644
index 000000000..9c440725c
--- /dev/null
+++ b/packages/core/src/utilities/isPlainObject.ts
@@ -0,0 +1,10 @@
+// see: https://github.com/mesqueeb/is-what/blob/88d6e4ca92fb2baab6003c54e02eedf4e729e5ab/src/index.ts
+
+function getType(payload: any): string {
+  return Object.prototype.toString.call(payload).slice(8, -1)
+}
+
+export default function isPlainObject(payload: any): payload is Record<string, any> {
+  if (getType(payload) !== 'Object') return false
+  return payload.constructor === Object && Object.getPrototypeOf(payload) === Object.prototype
+}
diff --git a/packages/core/src/utilities/mergeDeep.ts b/packages/core/src/utilities/mergeDeep.ts
index 884d0ff69..b28080b1f 100644
--- a/packages/core/src/utilities/mergeDeep.ts
+++ b/packages/core/src/utilities/mergeDeep.ts
@@ -1,12 +1,12 @@
 import { AnyObject } from '../types'
-import isObject from './isObject'
+import isPlainObject from './isPlainObject'
 
 export default function mergeDeep(target: AnyObject, source: AnyObject): AnyObject {
   const output = { ...target }
 
-  if (isObject(target) && isObject(source)) {
+  if (isPlainObject(target) && isPlainObject(source)) {
     Object.keys(source).forEach(key => {
-      if (isObject(source[key])) {
+      if (isPlainObject(source[key])) {
         if (!(key in target)) {
           Object.assign(output, { [key]: source[key] })
         } else {