diff --git a/fuzz/dns_message_checksig.c b/fuzz/dns_message_checksig.c
index 5987284a97..6480c76727 100644
--- a/fuzz/dns_message_checksig.c
+++ b/fuzz/dns_message_checksig.c
@@ -14,6 +14,7 @@
 #include <inttypes.h>
 #include <stdbool.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 #include <isc/buffer.h>
 #include <isc/commandline.h>