diff --git a/io/io_nonblock.c b/io/io_nonblock.c
index 3866018..69e1cfb 100644
--- a/io/io_nonblock.c
+++ b/io/io_nonblock.c
@@ -5,6 +5,6 @@
 
 void io_nonblock(int64 d) {
   io_entry* e=array_get(&io_fds,sizeof(io_entry),d);
-  fcntl(d,F_SETFL,fcntl(d,F_GETFL,0) | O_NDELAY);
-  if (e) e->nonblock=1;
+  if (fcntl(d,F_SETFL,fcntl(d,F_GETFL,0) | O_NDELAY)==0)
+    if (e) e->nonblock=1;
 }