diff --git a/packages/socket.io-client/support/rollup.config.umd.js b/packages/socket.io-client/support/rollup.config.umd.js
index 0d83056d77..44de2008f5 100644
--- a/packages/socket.io-client/support/rollup.config.umd.js
+++ b/packages/socket.io-client/support/rollup.config.umd.js
@@ -52,6 +52,7 @@ const prodBundle = {
         mangle: {
           properties: {
             regex: /^_/,
+            reserved: ["_placeholder"],
           },
         },
       }),