diff --git a/src/openvpn/error.h b/src/openvpn/error.h
index 972619fe..76308560 100644
--- a/src/openvpn/error.h
+++ b/src/openvpn/error.h
@@ -33,6 +33,8 @@
 
 #if _WIN32
 #include <windows.h>
+#else
+#include <unistd.h>
 #endif
 
 /* #define ABORT_ON_ERROR */