diff --git a/src/_utils.ts b/src/_utils.ts
new file mode 100644
index 0000000..52ac3ed
--- /dev/null
+++ b/src/_utils.ts
@@ -0,0 +1,16 @@
+// From sindresorhus/is-plain-obj
+// MIT License
+// Copyright (c) Sindre Sorhus <sindresorhus@gmail.com> (https://sindresorhus.com)
+export function isPlainObject(value: unknown): boolean {
+  if (value === null || typeof value !== "object") {
+    return false;
+  }
+  const prototype = Object.getPrototypeOf(value);
+  return (
+    (prototype === null ||
+      prototype === Object.prototype ||
+      Object.getPrototypeOf(prototype) === null) &&
+    !(Symbol.toStringTag in value) &&
+    !(Symbol.iterator in value)
+  );
+}
diff --git a/src/defu.ts b/src/defu.ts
index e942aec..271c023 100644
--- a/src/defu.ts
+++ b/src/defu.ts
@@ -1,3 +1,4 @@
+import { isPlainObject } from "./_utils";
 import type { Merger, DefuFn as DefuFunction, DefuInstance } from "./types";
 
 // Base function to apply defaults
@@ -7,7 +8,7 @@ function _defu<T>(
   namespace = ".",
   merger?: Merger,
 ): T {
-  if (!_isPlainObject(defaults)) {
+  if (!isPlainObject(defaults)) {
     return _defu(baseObject, {}, namespace, merger);
   }
 
@@ -30,7 +31,7 @@ function _defu<T>(
 
     if (Array.isArray(value) && Array.isArray(object[key])) {
       object[key] = [...value, ...object[key]];
-    } else if (_isPlainObject(value) && _isPlainObject(object[key])) {
+    } else if (isPlainObject(value) && isPlainObject(object[key])) {
       object[key] = _defu(
         value,
         object[key],
@@ -45,23 +46,6 @@ function _defu<T>(
   return object;
 }
 
-// From sindresorhus/is-plain-obj
-// MIT License
-// Copyright (c) Sindre Sorhus <sindresorhus@gmail.com> (https://sindresorhus.com)
-function _isPlainObject(value: unknown): boolean {
-  if (value === null || typeof value !== "object") {
-    return false;
-  }
-  const prototype = Object.getPrototypeOf(value);
-  return (
-    (prototype === null ||
-      prototype === Object.prototype ||
-      Object.getPrototypeOf(prototype) === null) &&
-    !(Symbol.toStringTag in value) &&
-    !(Symbol.iterator in value)
-  );
-}
-
 // Create defu wrapper with optional merger and multi arg support
 export function createDefu(merger?: Merger): DefuFunction {
   return (...arguments_) =>