diff --git a/common/string_util.h b/common/string_util.h
index 70aa1d16c..50ac60d64 100644
--- a/common/string_util.h
+++ b/common/string_util.h
@@ -25,6 +25,7 @@
 #ifndef _WIN32
 // this doesn't appear to affect linux-based systems..need feedback for _WIN64
 #include <fmt/format.h>
+#include <cctype>
 #endif
 
 #include "types.h"