diff --git a/collects/mzlib/port.rkt b/collects/mzlib/port.rkt
index 2a87650..81ce65d 100644
--- a/collects/mzlib/port.rkt
+++ b/collects/mzlib/port.rkt
@@ -710,7 +710,7 @@
          ;; write
          (lambda (str start end buffer? w/break?)
            (if (= start end)
-             #t
+             0
              (begin
                (resume-mgr)
                (call-with-semaphore