diff --git a/src/system4.c b/src/system4.c
index fd922c4..c7b002b 100644
--- a/src/system4.c
+++ b/src/system4.c
@@ -24,6 +24,7 @@
 #include <getopt.h>
 #include <time.h>
 #include <math.h>
+#include <limits.h>
 
 #include "system4.h"
 #include "system4/ain.h"