empty result for unassigned expressions

Hi, I wonder if the following result is the intended behavior of function getAssignedVar:

codetools::getAssignedVar(quote(y + 1))
#> [1] "y"

^{Created on 2023-07-02 with reprex v2.0.2}

I was thinking if it should return NULL as the result, since this expression does not get assigned to anything. Thanks!