diff --git a/package.json b/package.json
index 8dd908de..c431f7f8 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@bloxwap/hyperliquid",
-  "version": "0.1.5",
+  "version": "0.1.6",
   "description": "Blazing fast TypeScript Hyperliquid SDK.",
   "license": "MIT",
   "type": "module",
diff --git a/src/transport/websocket/_id.ts b/src/transport/websocket/_id.ts
index a7abf63c..9db6fc5c 100644
--- a/src/transport/websocket/_id.ts
+++ b/src/transport/websocket/_id.ts
@@ -85,6 +85,13 @@ export function normalize(value: unknown): unknown {
     const result: Record<string, unknown> = {};
     for (const key of Object.keys(value).sort()) {
       const item = normalize((value as Record<string, unknown>)[key]);
+      // Drop `undefined`-valued keys, matching what `JSON.stringify` does to this object on its
+      // way to the wire. Keeping them would leave the id and the echo carrying a field the server
+      // never receives and so can never echo back — and `isSubset` requires every key of the
+      // pending request to be present in the response, so such a subscription could never be
+      // matched to its own confirmation. A payload built as `{ dex: params.dex || undefined }`
+      // is the shape that hits this.
+      if (item === undefined) continue;
       // `__proto__` is the one key with a setter on `Object.prototype`: plain assignment would
       // invoke it and silently drop an own `__proto__` key a `JSON.parse`d payload can carry,
       // collapsing two logically distinct payloads onto the same id. Define it as a data
diff --git a/tests/transport/websocket/_id.test.ts b/tests/transport/websocket/_id.test.ts
index 1abd5ea5..fa306890 100644
--- a/tests/transport/websocket/_id.test.ts
+++ b/tests/transport/websocket/_id.test.ts
@@ -25,6 +25,17 @@ describe("requestToId", () => {
     assert(requestToId([1, 2]) !== requestToId([2, 1]));
   });
 
+  test("drops undefined-valued keys, matching what reaches the wire", () => {
+    // `JSON.stringify` omits an `undefined` value, so a payload built as
+    // `{ dex: params.dex || undefined }` goes out as `{"type":"allMids"}`. Keeping the key in the
+    // normalized form left the pending request demanding a field the server can never echo back,
+    // and `isSubset` then refused to match the subscription to its own confirmation — `allMids()`
+    // never resolved. The normalized form has to describe what was actually sent.
+    assertEquals(requestToId({ type: "allMids", dex: undefined }), requestToId({ type: "allMids" }));
+    assertEquals(requestToId({ type: "allMids", dex: undefined }), '{"type":"allMids"}');
+    assert(isSubset(normalize({ type: "allMids", dex: undefined }), { type: "allMids" }));
+  });
+
   test("keeps an own __proto__ key from a parsed payload", () => {
     const payload = JSON.parse('{"type":"l2Book","__proto__":{"coin":"BTC"}}') as Record<string, unknown>;
     assertEquals(requestToId(payload), '{"__proto__":{"coin":"BTC"},"type":"l2Book"}');