diff --git a/src/common/string_util.cpp b/src/common/string_util.cpp
index 38f6ad7d5..6f23382d5 100644
--- a/src/common/string_util.cpp
+++ b/src/common/string_util.cpp
@@ -1,5 +1,6 @@
 #include "string_util.h"
 #include <cstdio>
+#include <cctype>
 
 namespace StringUtil {
 
@@ -141,4 +142,4 @@ std::size_t Strlcpy(char* dst, const char* src, std::size_t size)
   return len;
 }
 
-} // namespace StringUtil
\ No newline at end of file
+} // namespace StringUtil