diff --git a/library/sha256.c b/library/sha256.c
index 102402e..dedc6b8 100644
--- a/library/sha256.c
+++ b/library/sha256.c
@@ -35,15 +35,20 @@
 
 #include "polarssl/sha256.h"
 
-#if defined(POLARSSL_FS_IO) || defined(POLARSSL_SELF_TEST)
+#include <string.h>
+
+#if defined(POLARSSL_FS_IO)
 #include <stdio.h>
 #endif
 
+#if defined(POLARSSL_SELF_TEST)
 #if defined(POLARSSL_PLATFORM_C)
 #include "polarssl/platform.h"
 #else
+#include <stdio.h>
 #define polarssl_printf printf
-#endif
+#endif /* POLARSSL_PLATFORM_C */
+#endif /* POLARSSL_SELF_TEST */
 
 /* Implementation that should never be optimized out by the compiler */
 static void polarssl_zeroize( void *v, size_t n ) {