diff --git a/index.js b/index.js
index ac2db1c..bf88bed 100644
--- a/index.js
+++ b/index.js
@@ -280,7 +280,11 @@ class Channel {
 
 module.exports = class Protomux {
   constructor(stream, { alloc, slave = false } = {}) {
-    if (stream.userData === null) stream.userData = this;
+    if (stream.userData) {
+      throw new Error("Mux already set for this stream");
+    }
+
+    stream.userData = this;
 
     this.isProtomux = true;
     this.stream = stream;