diff --git a/lib/init-7.scm b/lib/init-7.scm
index 83ed8d6f..748e7821 100644
--- a/lib/init-7.scm
+++ b/lib/init-7.scm
@@ -1199,9 +1199,12 @@
 
 (cond-expand
  (full-unicode
-  (define string-cursor-end string-size))
+  (define string-cursor-end string-size)
+  (define (string-offset->index str off)
+    (string-length (substring-cursor str 0 off))))
  (else
   (define (string-index->offset str i) i)
+  (define (string-offset->index str off) off)
   (define string-size string-length)
   (define substring-cursor substring)
   (define string-cursor-end string-length)