Nil can't be coerced into Fixnum in Rails

Question time!

I'm trying to figure out a way to calculate bonuses or penalties from a Character's Statistics, and then apply that method in other Models' views to calculate things like Fortitude saves, attack bonuses and so on. I've been working with a modifier for Constitution and here's what I've got so far.

The logic for calculating a Statistic bonus/penalty is (statistic - 10) / 2. So for a score of 14, we get 2, and for a score of 8, we get -1. Not bad so far!

So I define this method in Statistic controller:

def con_modifier
  (constitution.to_i - 10) / 2
end

And since I want to see it in my Character view, I wire it up in the Show method in Characters controller as such:

@con_modifier = @character.statistic.con_modifier

So I can see it in my Character view (where most of this information is displayed) and I know it's working so far. I've got a Fortitude model for Fortitude saves, and I'd like to use the con_modifier in the logic to calculate a total. How do I call a foreign model correctly this way? I have:

Here's the logic for calculating a total in my Fortitude model right now:

def total
  fortitude_base.to_i + ability.to_i + magic.to_i + misc.to_i
end

And I tried changing it to:

def total
  fortitude_base.to_i + @con_modifier + magic.to_i + misc.to_i
end

But then I get this error:

nil can't be coerced into Fixnum

So obviously it isn't calling up the correct information. I've noticed if I chage @con_modifier to @con_modifier.to_i, it calculates it at 0, because it's nil. Any ideas on how to call the number that actually exists? Do I need to define it in my Fortitudes controller as well, or can I simply define it in the Fortitude model and call it in the view that way?