diff --git a/sys/crypto/chacha20/chacha.c b/sys/crypto/chacha20/chacha.c
index e22c348d14b..8ce69226635 100644
--- a/sys/crypto/chacha20/chacha.c
+++ b/sys/crypto/chacha20/chacha.c
@@ -51,8 +51,8 @@ typedef struct chacha_ctx chacha_ctx;
   a = PLUS(a,b); d = ROTATE(XOR(d,a), 8); \
   c = PLUS(c,d); b = ROTATE(XOR(b,c), 7);
 
-static const char sigma[16] = "expand 32-byte k";
-static const char tau[16] = "expand 16-byte k";
+static const char sigma[16] __nonstring = "expand 32-byte k";
+static const char tau[16] __nonstring = "expand 16-byte k";
 
 LOCAL void
 chacha_keysetup(chacha_ctx *x,const u8 *k,u32 kbits)