Dirac's fine-structure formula for an "abnormally high" nuclear charge

In Dirac's fine-structure formula for the hydrogenlike atoms a critical role is played by the square root of the following expression: the unity minus the square of the product of the atomic number by the fine-structure constant (which is approximately equal to 1/137). Indeed, for a fictitious, ideal nucleus for which the above product is greater than or equal to one, the mentioned square root becomes imaginary, or zero. I prove in this Note that the origin of such theoretical "breaking down" is quite simple and is inherent in the special theory of relativity of classical physics.