diff --git a/msgpack/encode.ts b/msgpack/encode.ts
index cee2bac721a8..dd89a61de339 100644
--- a/msgpack/encode.ts
+++ b/msgpack/encode.ts
@@ -249,7 +249,9 @@ function encodeSlice(object: ValueType, byteParts: Uint8Array[]) {
   }
 
   // If object is a plain object
-  if (Object.getPrototypeOf(object) === Object.prototype) {
+  const prototype = Object.getPrototypeOf(object);
+
+  if (prototype === null || prototype === Object.prototype) {
     const numKeys = Object.keys(object).length;
 
     if (numKeys < FOUR_BITS) { // fixarray
diff --git a/msgpack/encode_test.ts b/msgpack/encode_test.ts
index a80949445c73..c808ed19c3b0 100644
--- a/msgpack/encode_test.ts
+++ b/msgpack/encode_test.ts
@@ -156,6 +156,9 @@ Deno.test("encode() handles maps", () => {
   const map0 = {};
   assertEquals(decode(encode(map0)), map0);
 
+  const mapNull = Object.create(null);
+  assertEquals(decode(encode(mapNull)), mapNull);
+
   const map1 = { "a": 0, "b": 2, "c": "three", "d": null };
   assertEquals(decode(encode(map1)), map1);