diff --git a/serial.c b/serial.c
index 681dc57..77b6bae 100644
--- a/serial.c
+++ b/serial.c
@@ -17,7 +17,7 @@
 #include <poll.h>
 #include <unistd.h>
 #else
-static const size_t write(int fd, void *buf, size_t len)
+static const size_t write(int fd, const void *buf, size_t len)
 {
 	return -1;
 }