diff --git a/lisp/emacs-lisp/bytecomp.el b/lisp/emacs-lisp/bytecomp.el
index aa9521e5a65..847965e6af6 100644
--- a/lisp/emacs-lisp/bytecomp.el
+++ b/lisp/emacs-lisp/bytecomp.el
@@ -2581,7 +2581,8 @@ byte-compile-flush-pending
 
 (defun byte-compile-preprocess (form &optional _for-effect)
   (let ((print-symbols-bare t))         ; Possibly redundant binding.
-    (setq form (macroexpand-all form byte-compile-macro-environment)))
+    (setq form (copy-tree
+                (macroexpand-all form byte-compile-macro-environment))))
   ;; FIXME: We should run byte-optimize-form here, but it currently does not
   ;; recurse through all the code, so we'd have to fix this first.
   ;; Maybe a good fix would be to merge byte-optimize-form into