diff --git a/lib/urldata.h b/lib/urldata.h
index 929ba96e4..775da1e19 100644
--- a/lib/urldata.h
+++ b/lib/urldata.h
@@ -275,20 +275,18 @@ struct ssl_connect_data {
   bool use;
   ssl_connection_state state;
   ssl_connect_state connecting_state;
-#ifdef USE_OPENSSL
+#if defined(USE_OPENSSL)
   /* these ones requires specific SSL-types */
   SSL_CTX* ctx;
   SSL*     handle;
   X509*    server_cert;
-#endif /* USE_OPENSSL */
-#ifdef USE_GNUTLS
+#elif defined(USE_GNUTLS)
   gnutls_session_t session;
   gnutls_certificate_credentials_t cred;
 #ifdef USE_TLS_SRP
   gnutls_srp_client_credentials_t srp_client_cred;
 #endif
-#endif /* USE_GNUTLS */
-#ifdef USE_MBEDTLS
+#elif defined(USE_MBEDTLS)
   mbedtls_ctr_drbg_context ctr_drbg;
   mbedtls_entropy_context entropy;
   mbedtls_ssl_context ssl;
@@ -299,8 +297,7 @@ struct ssl_connect_data {
   mbedtls_x509_crl crl;
   mbedtls_pk_context pk;
   mbedtls_ssl_config config;
-#endif /* USE_MBEDTLS */
-#ifdef USE_POLARSSL
+#elif defined(USE_POLARSSL)
   ctr_drbg_context ctr_drbg;
   entropy_context entropy;
   ssl_context ssl;
@@ -310,27 +307,22 @@ struct ssl_connect_data {
   x509_crt clicert;
   x509_crl crl;
   rsa_context rsa;
-#endif /* USE_POLARSSL */
-#ifdef USE_CYASSL
+#elif defined(USE_CYASSL)
   SSL_CTX* ctx;
   SSL*     handle;
-#endif /* USE_CYASSL */
-#ifdef USE_NSS
+#elif defined(USE_NSS)
   PRFileDesc *handle;
   char *client_nickname;
   struct SessionHandle *data;
   struct curl_llist *obj_list;
   PK11GenericObject *obj_clicert;
-#endif /* USE_NSS */
-#ifdef USE_GSKIT
+#elif defined(USE_GSKIT)
   gsk_handle handle;
   int iocport;
-#endif
-#ifdef USE_AXTLS
+#elif defined(USE_AXTLS)
   SSL_CTX* ssl_ctx;
   SSL*     ssl;
-#endif /* USE_AXTLS */
-#ifdef USE_SCHANNEL
+#elif defined(USE_SCHANNEL)
   struct curl_schannel_cred *cred;
   struct curl_schannel_ctxt *ctxt;
   SecPkgContext_StreamSizes stream_sizes;
@@ -341,13 +333,14 @@ struct ssl_connect_data {
   CURLcode recv_unrecoverable_err; /* schannel_recv had an unrecoverable err */
   bool recv_sspi_close_notify; /* true if connection closed by close_notify */
   bool recv_connection_closed; /* true if connection closed, regardless how */
-#endif /* USE_SCHANNEL */
-#ifdef USE_DARWINSSL
+#elif defined(USE_DARWINSSL)
   SSLContextRef ssl_ctx;
   curl_socket_t ssl_sockfd;
   bool ssl_direction; /* true if writing, false if reading */
   size_t ssl_write_buffered_length;
-#endif /* USE_DARWINSSL */
+#elif defined(USE_SSL)
+#error "SSL backend specific information missing from ssl_connect_data"
+#endif
 };
 
 struct ssl_config_data {