diff --git a/lib/binary.ts b/lib/binary.ts
index 3bc41b4..078bf4d 100644
--- a/lib/binary.ts
+++ b/lib/binary.ts
@@ -53,7 +53,7 @@ function _deconstructPacket(data, buffers) {
 
 export function reconstructPacket(packet, buffers) {
   packet.data = _reconstructPacket(packet.data, buffers);
-  packet.attachments = undefined; // no longer useful
+  delete packet.attachments; // no longer useful
   return packet;
 }
 
diff --git a/lib/index.ts b/lib/index.ts
index 1170d17..b3035d3 100644
--- a/lib/index.ts
+++ b/lib/index.ts
@@ -53,11 +53,15 @@ export class Encoder {
 
     if (obj.type === PacketType.EVENT || obj.type === PacketType.ACK) {
       if (hasBinary(obj)) {
-        obj.type =
-          obj.type === PacketType.EVENT
-            ? PacketType.BINARY_EVENT
-            : PacketType.BINARY_ACK;
-        return this.encodeAsBinary(obj);
+        return this.encodeAsBinary({
+          type:
+            obj.type === PacketType.EVENT
+              ? PacketType.BINARY_EVENT
+              : PacketType.BINARY_ACK,
+          nsp: obj.nsp,
+          data: obj.data,
+          id: obj.id,
+        });
       }
     }
     return [this.encodeAsString(obj)];
@@ -149,10 +153,9 @@ export class Decoder extends Emitter<{}, {}, DecoderReservedEvents> {
         throw new Error("got plaintext data when reconstructing a packet");
       }
       packet = this.decodeString(obj);
-      if (
-        packet.type === PacketType.BINARY_EVENT ||
-        packet.type === PacketType.BINARY_ACK
-      ) {
+      const isBinaryEvent = packet.type === PacketType.BINARY_EVENT;
+      if (isBinaryEvent || packet.type === PacketType.BINARY_ACK) {
+        packet.type = isBinaryEvent ? PacketType.EVENT : PacketType.ACK;
         // binary packet's json
         this.reconstructor = new BinaryReconstructor(packet);
 
diff --git a/test/arraybuffer.js b/test/arraybuffer.js
index 3e8884e..7103c80 100644
--- a/test/arraybuffer.js
+++ b/test/arraybuffer.js
@@ -89,4 +89,20 @@ describe("ArrayBuffer", () => {
     decoder.destroy(); // destroy before all data added
     expect(decoder.reconstructor.buffers.length).to.be(0); // expect that buffer is clean
   });
+
+  it("should not modify the input packet", () => {
+    const packet = {
+      type: PacketType.EVENT,
+      nsp: "/",
+      data: ["a", Uint8Array.of(1, 2, 3), Uint8Array.of(4, 5, 6)],
+    };
+
+    encoder.encode(packet);
+
+    expect(packet).to.eql({
+      type: PacketType.EVENT,
+      nsp: "/",
+      data: ["a", Uint8Array.of(1, 2, 3), Uint8Array.of(4, 5, 6)],
+    });
+  });
 });
diff --git a/test/helpers.js b/test/helpers.js
index 8a50adb..d3fe1f2 100644
--- a/test/helpers.js
+++ b/test/helpers.js
@@ -20,13 +20,10 @@ module.exports.test = (obj) => {
 // tests encoding of binary packets
 module.exports.test_bin = (obj) => {
   return new Promise((resolve) => {
-    const originalData = obj.data;
     const encodedPackets = encoder.encode(obj);
 
     const decoder = new parser.Decoder();
     decoder.on("decoded", (packet) => {
-      obj.data = originalData;
-      obj.attachments = undefined;
       expect(obj).to.eql(packet);
       resolve();
     });