diff --git a/src/util.h b/src/util.h
index 88e1ff59..9ee2b115 100644
--- a/src/util.h
+++ b/src/util.h
@@ -52,11 +52,11 @@ namespace nghttp2 {
 // The additional HTTP/2 protocol ALPN protocol identifier we also
 // supports for our applications to make smooth migration into final
 // h2 ALPN ID.
-#define NGHTTP2_H2_16_ALPN "\x5h2-16"
-#define NGHTTP2_H2_16 "h2-16"
+constexpr const char NGHTTP2_H2_16_ALPN[] = "\x5h2-16";
+constexpr const char NGHTTP2_H2_16[] = "h2-16";
 
-#define NGHTTP2_H2_14_ALPN "\x5h2-14"
-#define NGHTTP2_H2_14 "h2-14"
+constexpr const char NGHTTP2_H2_14_ALPN[] = "\x5h2-14";
+constexpr const char NGHTTP2_H2_14[] = "h2-14";
 
 namespace util {