Don't introduce useless mul

theano/tensor/opt.py

@@ -4120,6 +4120,9 @@ def local_sum_prod_mul_by_scalar(node):
             if new_op_input_nb_elements != 1:
                 mul_inputs.append(new_op_output)
 
+            if len(mul_inputs) == 1:
+                return mul_inputs
+            else:
                 return [T.mul(*mul_inputs)]
 
         if isinstance(node.op, T.Sum) and node_inps.owner and node_inps.owner.op == T.neg: