diff --git a/modules/emacs/undo/doctor.el b/modules/emacs/undo/doctor.el
new file mode 100644
index 000000000..d1fda36f4
--- /dev/null
+++ b/modules/emacs/undo/doctor.el
@@ -0,0 +1,7 @@
+;;; emacs/undo/doctor.el -*- lexical-binding: t; -*-
+
+(let* ((unwanted (if (modulep! +tree) 'undo-fu 'undo-tree))
+       (wanted   (if (modulep! +tree) 'undo-tree 'undo-fu)))
+  (when (doom-package-in-module-p unwanted :user)
+    (error! "User has installed %S, which is incompatible with %S" unwanted wanted)
+    (explain! "Set (or unset) this module's +tree flag to set up one or the other.")))