diff --git a/src/views/sus.lisp b/src/views/sus.lisp
index 244334e..b6c7d2b 100644
--- a/src/views/sus.lisp
+++ b/src/views/sus.lisp
@@ -2,6 +2,8 @@
 
 (defun load-form (lang form-file-name)
   "Load a Lisp file containing form definitions."
+  (check-type lang string)
+  (check-type form-file-name string)
   (let* ((relative-path (concatenate 'string "src/views/forms/" lang "/"))
          (full-path (uiop:merge-pathnames* relative-path (uiop:getcwd)))
          (form-path (uiop:merge-pathnames* form-file-name full-path)))