  The two-point string amplitude at tree level in flat spacetime reproduces the
expected expression for free particles. This has been proven by Erbin,
Maldacena and Skliros in [JHEP 07 (2019) 139] by two methods. Here, we provide
an alternative proof of this result. Our method consists in considering a
timelike Liouville direction as a regularization parameter, which suffices to
break time translation invariance and, at the same time, to stabilize the
residual conformal symmetry in the worldsheet. This allows taking a limit and
restoring Poincare symmetry in a controlled way, thus reproducing the correct
expression of the two-point amplitude.
