diff --git a/sys/include/hashes/sha256.h b/sys/include/hashes/sha256.h
index 9f9f24f81e95180f6118f365e0588c52fe2ecb9c..ff302e9e7540ed8a2627a76829a2a9bf40b9e905 100644
--- a/sys/include/hashes/sha256.h
+++ b/sys/include/hashes/sha256.h
@@ -49,6 +49,7 @@
 #define HASHES_SHA256_H
 
 #include <inttypes.h>
+#include <unistd.h>
 
 #ifdef __cplusplus
 extern "C" {