diff --git a/utils.rkt b/utils.rkt
index 53eb1c9..f4903db 100644
--- a/utils.rkt
+++ b/utils.rkt
@@ -222,7 +222,7 @@
 ;;; Reads a variable mapping from a string, such as the one which
 ;;; Org-mode produces from tables.
 (define (read-org-variable-mapping str)
-  (make-hash (unstringify-pairs (read-org-sexp str))))
+  (make-immutable-hash (unstringify-pairs (read-org-sexp str))))
 
 ;;; A synonym for read-org-variable-mapping.
 (define-syntax-rule (unorg str) (read-org-variable-mapping str))