diff --git a/module/mes/guile.mes b/module/mes/guile.mes
index bd2f3bb4..5b5051f4 100644
--- a/module/mes/guile.mes
+++ b/module/mes/guile.mes
@@ -148,7 +148,7 @@
                       (set! tell 0)
                       (set! peek-char
                             (lambda () (if (= tell end) (integer->char -1)
-                                           (string-ref string (- tell 1)))))
+                                           (string-ref string tell))))
                       (set! read-char
                             (lambda () (if (= tell end) (integer->char -1)
                                            (begin
diff --git a/tests/guile.test b/tests/guile.test
index 0af3b35e..1fa3b114 100755
--- a/tests/guile.test
+++ b/tests/guile.test
@@ -117,4 +117,10 @@ exit $?
                (with-input-from-string "X"
                  (lambda () (peek-char))))
 
+(pass-if-equal "open-input-string peek"
+               #\X
+               (let ((port (open-input-string "X")))
+                 (set-current-input-port port)
+                 (peek-char)))
+
 (result 'report)