diff --git a/wadext.h b/wadext.h
index 1621576..42cfe67 100644
--- a/wadext.h
+++ b/wadext.h
@@ -7,6 +7,7 @@
 
 #else // !_MSC_VER
 
+#include <ctype.h>
 #include <unistd.h>
 #include <sys/stat.h>