diff --git a/module/ice-9/sports.scm b/module/ice-9/sports.scm
index 37ea09221..86d9a5b06 100644
--- a/module/ice-9/sports.scm
+++ b/module/ice-9/sports.scm
@@ -309,13 +309,13 @@
        (else (fill-directly pos))))))
 
 (define (put-u8 port byte)
-  (when (port-random-access? port)
-    (flush-input port))
   (let* ((buf (port-write-buffer port))
          (bv (port-buffer-bytevector buf))
          (end (port-buffer-end buf)))
     (unless (<= 0 end (bytevector-length bv))
       (error "not an output port" port))
+    (when (and (eq? (port-buffer-cur buf) end) (port-random-access? port))
+      (flush-input port))
     (bytevector-u8-set! bv end byte)
     (set-port-buffer-end! buf (1+ end))
     (when (= (1+ end) (bytevector-length bv)) (flush-output port))))
@@ -324,14 +324,14 @@
                          (count (- (bytevector-length src) start)))
   (unless (<= 0 start (+ start count) (bytevector-length src))
     (error "invalid start/count" start count))
-  (when (port-random-access? port)
-    (flush-input port))
   (let* ((buf (port-write-buffer port))
          (bv (port-buffer-bytevector buf))
          (size (bytevector-length bv))
          (cur (port-buffer-cur buf))
          (end (port-buffer-end buf))
          (buffered (- end cur)))
+    (when (and (eq? cur end) (port-random-access? port))
+      (flush-input port))
     (cond
      ((<= size count)
       ;; The write won't fit in the buffer at all; write directly.