Runtime Error

Hello there, I'm having trouble with this stuff for some time - when I turn off the water quality part, the model runs fine to the end. However, when I turn on the WQ part, the model runs for a while and crashes on me. What happens is that the DEPTHB(KT,I) variable goes to NaN value and after that the model stops running when it reaches at the calculation of SROOUT in surface heat exchange module. I couldn't figure out what might be causing it. Any help is appreciated! take care, BD