diff --git a/cores/esp8266/base64.h b/cores/esp8266/base64.h
index 20f00f81fd..49a6cdfb01 100644
--- a/cores/esp8266/base64.h
+++ b/cores/esp8266/base64.h
@@ -25,6 +25,8 @@
 #ifndef CORE_BASE64_H_
 #define CORE_BASE64_H_
 
+#include <WString.h>
+
 class base64
 {
 public: