diff --git a/src/@dseries/horzcat.m b/src/@dseries/horzcat.m
index 47c1790f85ff09fef1345c9b55a67cfb5f97591b..dfff22655543b5bb63d529c4df70f67315e1dd18 100644
--- a/src/@dseries/horzcat.m
+++ b/src/@dseries/horzcat.m
@@ -43,7 +43,7 @@ switch nargin
   case 1
     o = varargin{1};
   otherwise
-    o = concatenate(varargin{1}, varargin{2});
+    o = concatenate(copy(varargin{1}), copy(varargin{2}));
     if nargin>2
         o = horzcat(o, varargin{3:end});
     end