diff --git a/src/output.cpp b/src/output.cpp
index 1926e6f156f53..d8a686d245e2f 100644
--- a/src/output.cpp
+++ b/src/output.cpp
@@ -9,6 +9,7 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#include <cwctype>
 #include <map>
 #include <sstream>
 #include <stack>