diff --git a/cores/esp8266/libc_replacements.c b/cores/esp8266/libc_replacements.c
index 7d6260d1e..21b451346 100644
--- a/cores/esp8266/libc_replacements.c
+++ b/cores/esp8266/libc_replacements.c
@@ -29,6 +29,7 @@
 #include <math.h>
 #include <limits.h>
 #include <errno.h>
+#include <string.h>
 
 #include "ets_sys.h"
 #include "os_type.h"