Derived variable stuck at 0 (unable to solve for fixed variable)

I'm supervising exercise sessions about LINGO and LP and today an exercise came up about investing. This is the intended solution:

MODEL:

SETS:
    year /t0 t1 t2 t3/: capital;
    investment /A B C D E/: allocated;
    table(investment, year): cash_flow;
ENDSETS

DATA:
    cash_flow = -1 0.5 1 0
            0 -1 0.5 1
            -1 1.2 0 0
            -1 0 0 1.9
            0 0 -1 1.5;
ENDDATA

max = capital(4);

! Capital(i) indicates the amount of money left after time i;

! Initial capital;
capital(1) = 100000 + @SUM(investment(j): allocated(j)*cash_flow(j, 1));

! Capital after one year is last year's capital plus interest
and the cash flows times investments of the current year;
@FOR(year(i) | i #GT# 1: capital(i) = 1.08*capital(i - 1)
    + @SUM(investment(j): allocated(j)*cash_flow(j, i)));

! Capital must be positive after each time period;
@FOR(year: capital >= 0);

! No more than $75000 may be invested in a single investment;
@FOR(investment: allocated <= 75000);
@FOR(investment: allocated >= 0);

END

One student tried to substitute the net cash flow per year by a variable to remove some code duplication. However, just by adding the variable net_cashflow to year's derived sets in the sets-section and adding the following constraint to the body:

@FOR(year(i): net_cashflow(i) = @SUM(investment(j): cash_flow(j, i)*allocated(j)));

LINGO's solution is now wrong. All net_cashflow(i)'s are set to 0 and as a result all allocated(j)'s are 0 too. When I try to force net_cashflow(1) = -100000 (as it should be in the optimal solution) with this constraint:

net_cashflow(1) = -100000;

I get the following error:

 [Error Code: 72]

    Unable to solve for fixed variable:
    NET_CASHFLOW( T0)
  in constraint:
    10
  Loosening the variable's bounds may help.

  The error occurred on or near the following line:
     31]net_cashflow(1) = -100000;

But even if I replace this constraint by very loose bounds (like net_cashflow(1) <= -1), LINGO tells me there is no feasible solution, as if 0 is the only feasible value for all net_cashflows. I've looked at this problem for over an hour now and still don't understand what's going on. Can anyone explain this?

(Since no one has responded and I have finally found the solution, it may be useful for future readers to post the answer myself:)

Apparently LINGO variables are non-negative by default. This issue was resolved by adding the constraint (or rather relaxation):

@FOR(year: @FREE(net_cashflow));