A student analyzes water draining from a conical tank, where the volume is given by $$V = \frac{1}{3} \pi r^2 h$$. Given that the radius r is proportional to the height h, he incorrectly assumes that r is constant when evaluating the rate of change of volume dV/dt at a moment when h = 5 m and dh/dt = -0.2 m/min. Identify the error in his process.