diff --git a/core/deps/coreio/coreio.cpp b/core/deps/coreio/coreio.cpp
index f7d8922ab..c9d0b351c 100644
--- a/core/deps/coreio/coreio.cpp
+++ b/core/deps/coreio/coreio.cpp
@@ -8,6 +8,7 @@
 #include <sstream>
 #include <string>
 #include <iomanip>
+#include <cctype>
 
 #define TRUE 1
 #define FALSE 0