From wikipedia:
In order to preserve the original rule:

for negative values of
we must use the recursive relation:

Thus:

However smaller negative values cannot be well defined in this way because

which is not well defined.
Note further that for
any definition of
is consistent with the rule because
for any
.
I believe that, by definition -12 = log2(02) = log2(1) = 0.
I would suggest going online to find a fuller explanation.
From wikipedia:
In order to preserve the original rule:

for negative values of
we must use the recursive relation:

Thus:

However smaller negative values cannot be well defined in this way because

which is not well defined.
Note further that for
any definition of
is consistent with the rule because
for any
.