diff --git a/serial/proxy.c b/serial/proxy.c
index 32878ba..d545e36 100644
--- a/serial/proxy.c
+++ b/serial/proxy.c
@@ -212,7 +212,7 @@ static int channel_write(GIOChannel *chan, char *buf, size_t size)
 	while (wbytes < size) {
 		written = write(fd, buf + wbytes, size - wbytes);
 
-		if (written)
+		if (written < 0)
 			return -errno;
 
 		wbytes += written;