Pyomo - Objective function as average value of param

Question

I am trying to use Pyomo for an LP problem and I would like the objective function to be the mean value of a particular parameter in my dataframe (which I'll call obj_param).

I had previously set this up like so:

model = ConcreteModel()
model.decision_var = Var(list(idx for idx in self.df.index), domain=NonNegativeReals)
model.obj = Objective(
    expr= -1 *  # because I want to maximize not minimize
          sum(model.decision_var[idx] * df.loc[idx,'obj_param'] for idx in df.index)
)

The decision_var here is a column of counts (like "acres of this crop" in the classic farmer problem) and the obj_param is the value of this "crop", so my objective (as written) multiplies the acres of the crop by it's value to maximize the total value.

This makes sense in the farmer problem, but what I'm actually trying to do in my case is to maximize the mean value of each acre. (Forgive the farmer metaphor, it becomes a bit strained here.)

To do this, I change my objective as follows:

model.obj = Objective(
    expr= -1 *  # because I want to maximize not minimize
          sum(model.decision_var[idx] * df.loc[idx,'obj_param'] for idx in df.index) /
          sum(model.decision_var[idx] for idx in df.index)
)

Conceptually this looks right to me, but now when I run it I get RuntimeError: Cannot write legal LP file. Objective 'obj' has nonlinear terms that are not quadratic.

I can vaguely understand what this error is saying, but I don't totally see how this equation is non-linear. Either way, more generally I'm asking: is it possible in pyomo to define the objective as an average in the way that I'm trying to do?

Thanks for any help!