Function: log Failed to evaluate variable

Hi,Sergei

I am getting the following error:
Log of negative
Function: log
Failed to evaluate variable
Variable: mod1.St, Defined as: ((mod1.p>=mod1.pea)*((cs-mod1.c)*(log((mod1.p/mod1.pea))*(exp(((-Ea)/(Rg*mod1.T)))*ka))))
Failed to evaluate expression
Expression: d((2*mod1.St*test(mod1.c)*pi*r)*(dvol),{test0})

Do you know how to fix this problem?

Thanks in advance for the comments