Fisher Transform: Use of the natural logarithm of negative number. How is it possible?

Question

I have the following equation, from http://www.mesasoftware.com/Papers/USING%20THE%20FISHER%20TRANSFORM.pdf

Because the parameters inside the log are (1+x)/(1-x), the output is always negative when x > 1.

But natural logarithms are only valid for positive number.

Anybody can explain that equation?

What am I missing?