diff --git a/main.scm b/main.scm
index a0d9bd6..d441318 100755
--- a/main.scm
+++ b/main.scm
@@ -151,11 +151,13 @@
    (pre-post-order*
     `((*text* .
        ,(lambda (_ str)
-          (->
-           str
-           (string-downcase)
-           (string-translate "/,:;\"[]{}()=+")
-           (string-translate " _." "---"))))
+          (if (string? str)
+              (->
+               str
+               (string-downcase)
+               (string-translate "/,:;\"[]{}()=+")
+               (string-translate " _." "---"))
+              str)))
       ,@alist-conv-rules*))
    (flatten)
    ((flip map) ->string)