diff --git a/src/openvpn/crypto_openssl.c b/src/openvpn/crypto_openssl.c
index de3878d..b6e2106 100644
--- a/src/openvpn/crypto_openssl.c
+++ b/src/openvpn/crypto_openssl.c
@@ -1412,7 +1412,8 @@
 ssl_tls1_PRF(const uint8_t *label, size_t label_len, const uint8_t *sec, size_t slen, uint8_t *out1,
              size_t olen)
 {
-    CRYPTO_tls1_prf(EVP_md5_sha1(), out1, olen, sec, slen, label, label_len, NULL, 0, NULL, 0);
+    return CRYPTO_tls1_prf(EVP_md5_sha1(), out1, olen, sec, slen,
+                           (const char *)label, label_len, NULL, 0, NULL, 0);
 }
 #elif !defined(LIBRESSL_VERSION_NUMBER) && !defined(ENABLE_CRYPTO_WOLFSSL)
 #if defined(__GNUC__) || defined(__clang__)