diff --git a/crypto/hkdf/hkdf.c b/crypto/hkdf/hkdf.c
index d80834d..f21cb42 100644
--- a/crypto/hkdf/hkdf.c
+++ b/crypto/hkdf/hkdf.c
@@ -55,7 +55,7 @@
 }
 
 int HKDF_expand(uint8_t *out_key, size_t out_len, const EVP_MD *digest,
-                uint8_t *prk, size_t prk_len, const uint8_t *info,
+                const uint8_t *prk, size_t prk_len, const uint8_t *info,
                 size_t info_len) {
   /* https://tools.ietf.org/html/rfc5869#section-2.3 */
   const size_t digest_len = EVP_MD_size(digest);
diff --git a/include/openssl/hkdf.h b/include/openssl/hkdf.h
index a484a30..bffb01e 100644
--- a/include/openssl/hkdf.h
+++ b/include/openssl/hkdf.h
@@ -50,7 +50,7 @@
  * |out_len| from the PRK |prk| and info |info| using |digest|, and outputs
  * the result to |out_key|. It returns one on success and zero on error. */
 OPENSSL_EXPORT int HKDF_expand(uint8_t *out_key, size_t out_len,
-                               const EVP_MD *digest, uint8_t *prk,
+                               const EVP_MD *digest, const uint8_t *prk,
                                size_t prk_len, const uint8_t *info,
                                size_t info_len);