diff --git a/HeraldTools/WeatherTools.cc b/HeraldTools/WeatherTools.cc
index 9f64c96..21e87cd 100644
--- a/src/HeraldTools/WeatherTools.cc
+++ b/src/HeraldTools/WeatherTools.cc
@@ -6,6 +6,7 @@
 #include <IoHerald.h>
 #include <fstream>
 #include <sstream>
+#include <unistd.h>
 #include <TMinuit.h>
 #include <TMath.h>