# non-linear model fit function not working

I have attempted to exercise a non-linear model fit to my data, `dadtAbs`, and got the following puzzle

``nlm =    NonlinearModelFit[     Transpose[{Table[t, {t, 1, tmax}],dadtAbs}],      (1 - aaa)*bbb*NSPI[bbb, ddd, population, t] *        (1 - ddd)*(population - NSPI[bbb, ddd, population, t]),      {aaa, bbb, ddd},      t] ``

where `dadtAbs` is a list, population is a known constant, `aaa`, `bbb`, `ddd` are the desired answers, and `t` is the variable.

When I queried `nlm`, I got

``[0.] ``

Here is the function `NSPI`:

``NSPI[alpha_, delta_, population_, tt_] :=    population/(1 + (population - 1)*Exp[-alpha*population*(1 - delta)*tt]) ``

population is a large constant, for example, 1000000