diff --git a/scsh/rw.scm b/scsh/rw.scm
index 317e449..c0ff5b3 100644
--- a/scsh/rw.scm
+++ b/scsh/rw.scm
@@ -16,9 +16,12 @@
   (let-optionals args ((fd/port (current-input-port))
 		       (start   0)
 		       (end     (string-length s)))
+    (if (bogus-substring-spec? s start end)
+      (error "Bad substring indices" s start end))
+
     (cond ((integer? fd/port)
 	   (let ((port (fdes->inport fd/port)))
-	     (set-port-buffering port bufpol/block (max (- end start) 0))
+	     (set-port-buffering port bufpol/block (- end start))
 	     (read-string!/partial port start end)))
 
           (else ; no differnce between fd/ports and s48 ports
@@ -51,9 +54,12 @@
   (let-optionals args ((fd/port (current-input-port))
 		       (start   0)
 		       (end     (string-length s)))
+    (if (bogus-substring-spec? s start end)
+      (error "Bad substring indices" s start end))
+
     (cond ((integer? fd/port)
 	   (let ((port (fdes->inport fd/port)))
-	     (set-port-buffering port bufpol/block (max (- end start) 0))
+	     (set-port-buffering port bufpol/block (- end start))
 	     (read-string! port start end)))
 
 	  (else ; no differnce between fd/port and s48 ports
@@ -79,9 +85,12 @@
   (let-optionals args ((fd/port (current-output-port))
 		       (start 0)
 		       (end (string-length s)))
+    (if (bogus-substring-spec? s start end)
+	(error "Bad substring indices" s start end))
+
     (cond ((integer? fd/port)
 	   (let ((port (fdes->outport fd/port)))
-	     (set-port-buffering port bufpol/block (max (- end start) 0))
+	     (set-port-buffering port bufpol/block (- end start))
 	     (write-string/partial s port start end)))
 	  (else 
 	   ;; the only way to implement this, would be to use 
@@ -98,8 +107,11 @@ See the RELEASE file for details")))))
   (let-optionals args ((fd/port (current-output-port))
 		       (start   0)
 		       (end     (string-length s)))
+    (if (bogus-substring-spec? s start end)
+	(error "Bad substring indices" s start end))
+
     (cond ((integer? fd/port)
 	   (let ((port (fdes->outport fd/port)))
-	     (set-port-buffering port bufpol/block (max (- end start) 0))
+	     (set-port-buffering port bufpol/block (- end start))
 	     (write-string s port start end)))
 	  (else (write-block s start (- end start) fd/port)))))