diff --git a/lib/dns/gen.c b/lib/dns/gen.c
index 472ce43f91..201429f132 100644
--- a/lib/dns/gen.c
+++ b/lib/dns/gen.c
@@ -43,6 +43,10 @@
 #include "gen-unix.h"
 #endif
 
+#ifndef ULLONG_MAX
+#define ULLONG_MAX (~0ULL)
+#endif
+
 #define INSIST(cond) \
 	if (!(cond)) { \
 		fprintf(stderr, "%s:%d: INSIST(%s)\n", \