Index: subversion/libsvn_ra_serf/util.c
===================================================================
--- subversion/libsvn_ra_serf/util.c	(revision 1510435)
+++ subversion/libsvn_ra_serf/util.c	(working copy)
@@ -2434,6 +2434,10 @@ svn_ra_serf__error_on_status(serf_status_line slin
                       "server or an intermediate proxy does not accept "
                       "chunked encoding. Try setting 'http-chunked-requests' "
                       "to 'auto' or 'no' in your client configuration."));
+      case 501:
+        return svn_error_createf(SVN_ERR_UNSUPPORTED_FEATURE, NULL,
+                                 _("The requested feature is not supported by "
+                                   "'%s'"), path);
     }
 
   if (sline.code >= 300)