diff --git a/config/amanda/ipv6.m4 b/config/amanda/ipv6.m4
index 9d76979339..d91ab95ab9 100644
--- a/config/amanda/ipv6.m4
+++ b/config/amanda/ipv6.m4
@@ -85,7 +85,7 @@ AC_DEFUN([AMANDA_CHECK_IPV6],
 #include <sys/socket.h>
 #include <errno.h>
 
-main()
+int main(void)
 {
    int aa;
    aa = socket(AF_INET6, SOCK_STREAM, 0);