  In a recent paper [Phys. Lett. A 383 (2019) 2836; arXiv:1906.05121
[quant-ph]], Bagarello, Gargano, and Roccati have claimed that no
square-integrable vacuum exists in quantizing the Bateman oscillator model. In
this paper, we rebut their claim by actually deriving the square-integrable
vacuum eigenfunction using a common procedure. We see that no problems occur in
quantizing the Bateman oscillator model.
