Log of a Negative Market-Book Ratio in a Regression Model As noted in this post, the book value of a company is its assets minus its liabilities.

I am reading this paper: "The price of sin: the effect of social norms on market" by Hong and Kacperczyk. One of the paper's hypothesis is that institutional investors are less likely to hold sin stocks. To demonstrate this, the following initial (Fama-Macbeth) regression was run as a benchmark:

$$IO \sim Logsize + Beta + LOGMB + PRINV + STD+ RET $$

IO is institutional ownership
Logsize is the market cap
Beta is the beta of the sector the stock belongs
LOGMB is the log of the market to book ratio.
PRINV is the inverse of price
STD is standard deviation of monthly return
RET is log of arithmetic of previous year's monthly return.

My question here is that, because the Market book ratio can be negative, how can we take log of this? 

It is not specified in the paper how this is treated. I have the strong suspicion that it is likely any company with negative book value for whatever reason is ignored. Surely this would introduce some sort of bias. Does anyone know what is the standard practice?
Well the problem can be dodged by the use of some simple solution of just adding a constant to all values which circumvent issues of taking logarithms of negative numbers. Usually the mean is used as a centering point. If the mean is negative though some other number must be chosen.

Interpretation of such a transformed variable would be different from its regular logged counter part.

These are just some hunches on what could have been done. Its odd that methods aren't mentioned for this case.