diff --git a/ext/openssl/ossl_ssl.c b/ext/openssl/ossl_ssl.c
index 47bbac92c5..81b5a40157 100644
--- a/ext/openssl/ossl_ssl.c
+++ b/ext/openssl/ossl_ssl.c
@@ -869,7 +869,7 @@ ossl_sslctx_setup(VALUE self)
     val = rb_iv_get(self, "@alpn_protocols");
     if (!NIL_P(val)) {
 	VALUE rprotos = ssl_encode_npn_protocols(val);
-	SSL_CTX_set_alpn_protos(ctx, StringValueCStr(rprotos), RSTRING_LEN(rprotos));
+	SSL_CTX_set_alpn_protos(ctx, (const unsigned char *)StringValueCStr(rprotos), RSTRING_LENINT(rprotos));
 	OSSL_Debug("SSL ALPN values added");
     }
     if (RTEST(rb_iv_get(self, "@alpn_select_cb"))) {