In the Convergecast Scheduling Problem, it is required to find in the communication graph an oriented spanning aggregation tree with a root in a base station and the arcs oriented to the root and to build a conflict-free min-length schedule for aggregating data along the arcs of the aggregation tree. This problem is NP-hard in general, however, if the communication graph is a unit square grid in each node of which there is a sensor and in which a data packet is transmitted along any edge during a one-time slot, the problem is polynomially solvable. In this paper, we consider a communication graph in the form of a square grid with rectangular obstacles impenetrable by the messages. In our previous paper, we proposed a polynomial algorithm for constructing a feasible schedule and intensive numerical experiment allowed us to make a hypothesis that the algorithm constructs an optimal solution. In this paper, we present a counterexample and prove that the proposed algorithm constructs a schedule of length at most one time round longer than the optimal schedule.