diff --git a/Source/GSFormat.m b/Source/GSFormat.m
index a1fecf679..53c2bc701 100644
--- a/Source/GSFormat.m
+++ b/Source/GSFormat.m
@@ -67,6 +67,7 @@
 #include <unistd.h>
 #include <sys/types.h>
 #include <fcntl.h>
+#include <wchar.h>
 #include <stdio.h>
 #ifdef HAVE_UINTMAX_T
 #include <stdint.h>