diff --git a/src/sha1.c b/src/sha1.c
index 2b922482..fcc8de13 100644
--- a/src/sha1.c
+++ b/src/sha1.c
@@ -194,6 +194,7 @@ static void bin2str(char *to, const unsigned char *p, size_t len) {
 #include "errors.h"
 
 #include <stdint.h>
+#include <string.h>
 
 #include <Rinternals.h>
 
diff --git a/src/sha256.c b/src/sha256.c
index 2c7cb014..f8c510a7 100644
--- a/src/sha256.c
+++ b/src/sha256.c
@@ -203,6 +203,7 @@ static void bin2str(char *to, const unsigned char *p, size_t len) {
 #include "errors.h"
 
 #include <stdint.h>
+#include <string.h>
 
 #include <Rinternals.h>