diff --git a/src/3rdparty/squirrel/squirrel/sqbaselib.cpp b/src/3rdparty/squirrel/squirrel/sqbaselib.cpp
index 25179de297..5281ec2aba 100644
--- a/src/3rdparty/squirrel/squirrel/sqbaselib.cpp
+++ b/src/3rdparty/squirrel/squirrel/sqbaselib.cpp
@@ -16,6 +16,7 @@
 #include "sqclosure.h"
 #include "sqclass.h"
 #include <stdarg.h>
+#include <ctype.h>
 
 #include "../../../safeguards.h"