Getting error in MEX file execution

I'm doing parameter tuning for an equation of state using the genetic algorithm function (ga). When I run the equation of state file as I coded it in MATLAB, it works fine. When I compile and package the file as a MEX file, it gives me the following error:

Error using
coder.internal.error (line
17)
Domain error. To compute
complex results from real x,
use 'log(complex(x))'.

Error in log (line 13)
        coder.internal.error('Coder:toolbox:ElFunDomainError',mfilename);

Error in EOSfunction (line 552)
logPHI = MUres_kT - log(Z);

What could be causing this? I have checks to make sure that Z is always real and non-negative, so log(Z) should never be causing a problem.

[Walter Roberson]

As an experiment, just above that logPHI line, add

assert(Z > 0);

and see what happens.

[Caleb]

Didn't work. Here's the error statement:

Error using EOSfile (line 552)
  Assertion failed.

EDIT: I guess "Assertion failed" means that Z is returning a negative number. That was the fix. Thank you, Walter