A fast and accurate estimation of the arsenic concentration in an electrochemical sensor is of great importance in optimization of the sensor. Legendre wavelet method was used for this purpose (Electrochimica Acta, 266(2018), 27-33), however, after a careful looking through the solution process, the model can only lead to a zero solution, this paradox can be successfully solved by a fractal modification of the surface coverage model. A simple solution process to the fractal model is elucidated. (C) 2018 Elsevier Ltd. All rights reserved.