diff --git a/racket/src/rktio/rktio_fd.c b/racket/src/rktio/rktio_fd.c
index b2b89f86c0..edb27145ee 100644
--- a/racket/src/rktio/rktio_fd.c
+++ b/racket/src/rktio/rktio_fd.c
@@ -1229,7 +1229,7 @@ intptr_t rktio_write(rktio_t *rktio, rktio_fd_t *rfd, const char *buffer, intptr
 
         if (rfd->oth) {
           if (rfd->oth->needflush) {
-            /* Not flushed, but we haven't promised not to block: */
+	    /* Not flushed, but we haven't promised to make progress: */
             flushed = 1;
           } else {
             WaitForSingleObject(rfd->oth->lock_sema, INFINITE);
@@ -1285,6 +1285,9 @@ intptr_t rktio_write(rktio_t *rktio, rktio_fd_t *rfd, const char *buffer, intptr
           winwrote = 0;
         }
 
+	if (ok && winwrote && rfd->oth)
+	  rfd->oth->needflush = 1;
+
         if (ok)
           out_len = winwrote;
       } else {