diff --git a/packages/babel-register/src/node.js b/packages/babel-register/src/node.js
index e58a662673..ad3ddb8f78 100644
--- a/packages/babel-register/src/node.js
+++ b/packages/babel-register/src/node.js
@@ -97,6 +97,8 @@ register({
 });
 
 export default function register(opts?: Object = {}) {
+  // Clone to avoid mutating the arguments object with the 'delete's below.
+  opts = Object.assign({}, opts);
   if (opts.extensions) hookExtensions(opts.extensions);
 
   if (opts.cache === false) cache = null;