diff --git a/similar/main/inferno.cpp b/similar/main/inferno.cpp
index 23cdf658a..24ac348f2 100644
--- a/similar/main/inferno.cpp
+++ b/similar/main/inferno.cpp
@@ -51,6 +51,7 @@ char copyright[] = "DESCENT II  COPYRIGHT (C) 1994-1996 PARALLAX SOFTWARE CORPOR
 #endif
 
 #include <cctype>
+#include <locale>
 #include "pstypes.h"
 #include "strutil.h"
 #include "console.h"