diff --git a/lib/chibi/io/io.scm b/lib/chibi/io/io.scm
index 6789c109..73597a53 100644
--- a/lib/chibi/io/io.scm
+++ b/lib/chibi/io/io.scm
@@ -349,9 +349,10 @@
           ((>= (- len offset) n)
            (string-copy! str start buf offset (+ offset n))
            (set! offset (+ offset n))
-           n)
+           (- (string-index->offset str end)
+              (string-index->offset str start)))
           (else
-           (string-copy! str start buf offset (+ offset len))
+           (string-copy! str start buf offset len)
            (let lp ((i (+ start (- len offset))))
              (set! buf (generator))
              (cond
@@ -359,15 +360,17 @@
                (set! buf "")
                (set! len 0)
                (set! offset 0)
-               (- i start))
+               (- (string-index->offset str i)
+                  (string-index->offset str start)))
               (else
                (set! len (string-length buf))
                (set! offset 0)
                (cond
                 ((>= (- len offset) (- n i))
-                 (string-copy! str i buf offset (+ offset (- n i)))
-                 (set! offset (+ offset (- n i)))
-                 n)
+                 (string-copy! str i buf 0 (- n i))
+                 (set! offset (- n i))
+                 (- (string-index->offset str end)
+                    (string-index->offset str start)))
                 (else
                  (string-copy! str i buf offset len)
                  (lp (+ i (- len offset)))))))))))))))
diff --git a/tests/io-tests.scm b/tests/io-tests.scm
index 74b631e2..60ed71a1 100644
--- a/tests/io-tests.scm
+++ b/tests/io-tests.scm
@@ -89,6 +89,30 @@
         (display "abc" out)
         (close-output-port out)))))
 
+(define (strings->input-port str-ls)
+  (make-generated-input-port
+   (lambda ()
+     (and (pair? str-ls)
+          (let ((res (car str-ls)))
+            (set! str-ls (cdr str-ls))
+            res)))))
+
+(test "abcdef" (read-line (strings->input-port '("abcdef"))))
+(test "abcdef" (read-line (strings->input-port '("abc" "def"))))
+(test "abcdef" (read-line (strings->input-port '("a" "b" "c" "d" "e" "f"))))
+(test "日本語" (read-line (strings->input-port '("日本語"))))
+(test "日本語" (read-line (strings->input-port '("日" "本" "語"))))
+(test "abc"
+    (let ((in (strings->input-port
+               (list "日本語" (make-string 4087 #\-) "abc"))))
+      (read-string 4090 in)
+      (read-line in)))
+(test "abc"
+    (let ((in (strings->input-port
+               (list "日本語" (make-string 4093 #\-) "abc"))))
+      (read-string 4096 in)
+      (read-line in)))
+
 (let ((in (make-custom-binary-input-port
            (let ((i 0))
              (lambda (bv start end)