diff --git a/package-lock.json b/package-lock.json
index 98f324048..e5b22d227 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -12,7 +12,6 @@
         "@octokit/endpoint": "^9.0.0",
         "@octokit/request-error": "^5.0.0",
         "@octokit/types": "^12.0.0",
-        "is-plain-object": "^5.0.0",
         "universal-user-agent": "^6.0.0"
       },
       "devDependencies": {
@@ -28,7 +27,7 @@
         "glob": "^10.2.4",
         "jest": "^29.0.0",
         "lolex": "^6.0.0",
-        "prettier": "3.0.3",
+        "prettier": "3.1.0",
         "semantic-release-plugin-update-version-in-files": "^1.0.0",
         "string-to-arraybuffer": "^1.0.2",
         "ts-jest": "^29.0.0",
@@ -5793,9 +5792,9 @@
       }
     },
     "node_modules/prettier": {
-      "version": "3.0.3",
-      "resolved": "https://registry.npmjs.org/prettier/-/prettier-3.0.3.tgz",
-      "integrity": "sha512-L/4pUDMxcNa8R/EthV08Zt42WBO4h1rarVtK0K+QJG0X187OLo7l699jWw0GKuwzkPQ//jMFA/8Xm6Fh3J/DAg==",
+      "version": "3.1.0",
+      "resolved": "https://registry.npmjs.org/prettier/-/prettier-3.1.0.tgz",
+      "integrity": "sha512-TQLvXjq5IAibjh8EpBIkNKxO749UEWABoiIZehEPiY4GNpVdhaFKqSTu+QrlU6D2dPAfubRmtJTi4K4YkQ5eXw==",
       "dev": true,
       "bin": {
         "prettier": "bin/prettier.cjs"
diff --git a/package.json b/package.json
index f95dea173..968114e90 100644
--- a/package.json
+++ b/package.json
@@ -25,7 +25,6 @@
     "@octokit/endpoint": "^9.0.0",
     "@octokit/request-error": "^5.0.0",
     "@octokit/types": "^12.0.0",
-    "is-plain-object": "^5.0.0",
     "universal-user-agent": "^6.0.0"
   },
   "devDependencies": {
@@ -41,7 +40,7 @@
     "glob": "^10.2.4",
     "jest": "^29.0.0",
     "lolex": "^6.0.0",
-    "prettier": "3.0.3",
+    "prettier": "3.1.0",
     "semantic-release-plugin-update-version-in-files": "^1.0.0",
     "string-to-arraybuffer": "^1.0.2",
     "ts-jest": "^29.0.0",
diff --git a/src/fetch-wrapper.ts b/src/fetch-wrapper.ts
index 68bb78001..ef41903cf 100644
--- a/src/fetch-wrapper.ts
+++ b/src/fetch-wrapper.ts
@@ -1,4 +1,4 @@
-import { isPlainObject } from "is-plain-object";
+import { isPlainObject } from "./is-plain-object";
 import { RequestError } from "@octokit/request-error";
 import type { EndpointInterface } from "@octokit/types";
 
diff --git a/src/is-plain-object.ts b/src/is-plain-object.ts
new file mode 100644
index 000000000..2addec5d5
--- /dev/null
+++ b/src/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/test/is-plain-object.test.ts b/test/is-plain-object.test.ts
new file mode 100644
index 000000000..962c915b2
--- /dev/null
+++ b/test/is-plain-object.test.ts
@@ -0,0 +1,31 @@
+import { isPlainObject } from "../src/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);
+  });
+});