diff --git a/modules/pinyinhelper/pinyinlookup.h b/modules/pinyinhelper/pinyinlookup.h
index 85e656a..8b54953 100644
--- a/modules/pinyinhelper/pinyinlookup.h
+++ b/modules/pinyinhelper/pinyinlookup.h
@@ -20,6 +20,7 @@
 #define _PINYINHELPER_PINYINLOOKUP_H_
 
 #include <cstdint>
+#include <string>
 #include <unordered_map>
 #include <vector>