'NaN in Gamma Calculation' while estimating soil NHx from EPIC output

I am getting this runtime error in the log file few seconds after running CMAQv5.2.1 with NH3-bidi option on:

What would cause ‘gam’ (and MNHx) to be NaN? It looks like Gamma is a derived quantity in CMAQ since I do not see it in the EPIC input files.