diff --git a/modules/proxy/mod_proxy.h b/modules/proxy/mod_proxy.h
index f55f42899e..3e63e24560 100644
--- a/modules/proxy/mod_proxy.h
+++ b/modules/proxy/mod_proxy.h
@@ -806,8 +806,9 @@ PROXY_DECLARE(int) ap_proxy_post_request(proxy_worker *worker,
  * @param url     request url
  * @param proxyname are we connecting directly or via a proxy
  * @param proxyport proxy host port
- * @param server_portstr Via headers server port
- * @param server_portstr_size size of the server_portstr buffer
+ * @param server_portstr Via headers server port, must be non-NULL
+ * @param server_portstr_size size of the server_portstr buffer; must
+ * be at least one, even if the protocol doesn't use this
  * @return         OK or HTTP_XXX error
  */
 PROXY_DECLARE(int) ap_proxy_determine_connection(apr_pool_t *p, request_rec *r,
diff --git a/modules/proxy/proxy_util.c b/modules/proxy/proxy_util.c
index 9eb6aa80b6..917ee8db38 100644
--- a/modules/proxy/proxy_util.c
+++ b/modules/proxy/proxy_util.c
@@ -2329,8 +2329,9 @@ ap_proxy_determine_connection(apr_pool_t *p, request_rec *r,
 
     /* Get the server port for the Via headers */
     server_port = ap_get_server_port(r);
+    AP_DEBUG_ASSERT(server_portstr_size > 0);
     if (ap_is_default_port(server_port, r)) {
-        strcpy(server_portstr,"");
+        server_portstr[0] = '\0';
     }
     else {
         apr_snprintf(server_portstr, server_portstr_size, ":%d",