Added missing argument.

src/@dseries/backcast.m

@@ -31,7 +31,12 @@ function q = backcast(o, p, diff)  % --*-- Unitary tests --*--
 % along with Dynare.  If not, see <http://www.gnu.org/licenses/>.
 
 q = copy(o);
-q.backcast_(p);
+
+if nargin>2
+    q.backcast_(p, diff);
+else
+    q.backcast_(p);
+end
 
 return