diff --git a/common/string_util.h b/common/string_util.h
index 6582ef975..78fbe66f2 100644
--- a/common/string_util.h
+++ b/common/string_util.h
@@ -28,6 +28,7 @@
 #endif
 
 #ifdef _WINDOWS
+#include <ctype.h>
 #include <functional>
 #include <algorithm>
 #endif