diff --git a/package-lock.json b/package-lock.json
index 38221fb5..c18856ca 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -10,7 +10,6 @@
       "license": "MIT",
       "dependencies": {
         "@octokit/types": "^12.0.0",
-        "is-plain-object": "^5.0.0",
         "universal-user-agent": "^6.0.0"
       },
       "devDependencies": {
@@ -4075,6 +4074,7 @@
       "version": "5.0.0",
       "resolved": "https://registry.npmjs.org/is-plain-object/-/is-plain-object-5.0.0.tgz",
       "integrity": "sha512-VRSzKkbMm5jMDoKLbltAkFQ5Qr7VDiTFGXxYFXXowVj387GeGNOCsOH6Msy00SGZ3Fp84b1Naa1psqgcCIEP5Q==",
+      "dev": true,
       "engines": {
         "node": ">=0.10.0"
       }
diff --git a/package.json b/package.json
index 7db2b58c..e0e32a7a 100644
--- a/package.json
+++ b/package.json
@@ -35,7 +35,6 @@
   },
   "dependencies": {
     "@octokit/types": "^12.0.0",
-    "is-plain-object": "^5.0.0",
     "universal-user-agent": "^6.0.0"
   },
   "release": {
diff --git a/src/util/is-plain-object.ts b/src/util/is-plain-object.ts
new file mode 100644
index 00000000..2addec5d
--- /dev/null
+++ b/src/util/is-plain-object.ts
@@ -0,0 +1,17 @@
+export function isPlainObject(value: unknown): value is Object {
+  if (typeof value !== "object" || value === null) return false;
+
+  if (Object.prototype.toString.call(value) !== "[object Object]") return false;
+
+  const proto = Object.getPrototypeOf(value);
+  if (proto === null) return true;
+
+  const Ctor =
+    Object.prototype.hasOwnProperty.call(proto, "constructor") &&
+    proto.constructor;
+  return (
+    typeof Ctor === "function" &&
+    Ctor instanceof Ctor &&
+    Function.prototype.call(Ctor) === Function.prototype.call(value)
+  );
+}
diff --git a/src/util/merge-deep.ts b/src/util/merge-deep.ts
index f5019c09..9b38cfc3 100644
--- a/src/util/merge-deep.ts
+++ b/src/util/merge-deep.ts
@@ -1,4 +1,4 @@
-import { isPlainObject } from "is-plain-object";
+import { isPlainObject } from "./is-plain-object";
 
 export function mergeDeep(defaults: any, options: any): object {
   const result = Object.assign({}, defaults);
diff --git a/test/is-plaint-object.test.ts b/test/is-plaint-object.test.ts
new file mode 100644
index 00000000..44071449
--- /dev/null
+++ b/test/is-plaint-object.test.ts
@@ -0,0 +1,31 @@
+import { isPlainObject } from "../src/util/is-plain-object";
+
+describe("isPlainObject", () => {
+  function Foo() {
+    // @ts-ignore
+    this.a = 1;
+  }
+
+  it("isPlainObject(NaN)", () => {
+    expect(isPlainObject(NaN)).toBe(false);
+  });
+  it("isPlainObject([1, 2, 3])", () => {
+    expect(isPlainObject([1, 2, 3])).toBe(false);
+  });
+  it("isPlainObject(null)", () => {
+    expect(isPlainObject(null)).toBe(false);
+  });
+  it("isPlainObject({ 'x': 0, 'y': 0 })", () => {
+    expect(isPlainObject({ x: 0, y: 0 })).toBe(true);
+  });
+  it("isPlainObject(Object.create(null))", () => {
+    expect(isPlainObject(Object.create(null))).toBe(true);
+  });
+  it("isPlainObject(Object.create(new Foo()))", () => {
+    // @ts-ignore
+    expect(isPlainObject(Object.create(new Foo()))).toBe(false);
+  });
+  it("isPlainObject(Object.create(new Date()))", () => {
+    expect(isPlainObject(Object.create(new Date()))).toBe(false);
+  });
+});