I log-transformed a ratio variable (sem), which ranged from 5.73e-10 to .0001021 (mean: 1.72e-06, s.d.: 2.15e-06).

Variable | Obs Mean Std. Dev. Min Max
-------------+--------------------------------------------------------
sem | 9176 1.72e-06 2.15e-06 5.73e-10 .0001021

To prevent negative numbers after log transformation, I added 1 to the original variable like below.
gen lsem=log(sem+1)

But, the log transformation was failed as follows. All values remained unchanged.

Variable | Obs Mean Std. Dev. Min Max
-------------+--------------------------------------------------------
sem | 9176 1.72e-06 2.15e-06 5.73e-10 .0001021
lsem | 9176 1.72e-06 2.15e-06 5.73e-10 .0001021

What was wrong? How can I log-transform this variable?