diff --git a/module/srfi/srfi-13.mes b/module/srfi/srfi-13.mes
index bf6a06e9..859e5160 100644
--- a/module/srfi/srfi-13.mes
+++ b/module/srfi/srfi-13.mes
@@ -54,14 +54,20 @@
                         (list (list->string (list-head lst (- (length lst) (length rest)))))))))))
 
 (define (string-take s n)
-  (list->string (list-head (string->list s) n)))
+  (cond ((zero? n) s)
+        ((> n 0) (list->string (list-head (string->list s) n)))
+        (else (error "string-take: not supported: n=" n))))
 
 (define (string-drop s n)
-  (list->string (list-tail (string->list s) n)))
+  (cond ((zero? n) s)
+        ((> n 0) (list->string (list-tail (string->list s) n)))
+        (else s (error "string-drop: not supported: (n s)=" (cons n s)))))
 
 (define (string-drop-right s n)
-  (let ((length (string-length s)))
-    (list->string (list-head (string->list s) (- length n)))))
+  (cond ((zero? n) s)
+        ((> n 0) (let ((length (string-length s)))
+                   (list->string (list-head (string->list s) (- length n)))))
+        (else (error "string-drop-right: not supported: n=" n))))
 
 (define (string-delete pred s)
   (let ((p (if (procedure? pred) pred
diff --git a/tests/srfi-13.test b/tests/srfi-13.test
index 5fc9d689..1e6a9851 100755
--- a/tests/srfi-13.test
+++ b/tests/srfi-13.test
@@ -56,4 +56,10 @@ exit $?
     "f-o-o-:"
   (list->string (string-fold-right (lambda (e p) (cons e (cons #\- p))) '(#\:) "foo")))
 
+(pass-if-equal "string-drop" "bar"
+  (string-drop "foobar" 3))
+
+(pass-if-equal "string-drop-right" "foo"
+  (string-drop-right "foobar" 3))
+
 (result 'report)