[Python] Docplex: KeyError: (-1, 0) when attempting to add a constraint that makes the first...

Part of my model states that the initial value of a time series should always equal the last. When modelling this for just one time series, the constraint is added no problem.

m.add_constraint(var_1[-1] == var_1[0])


But I am trying to add it to multiple time series with the following:

var_1 = m.continuous_var_dict([(i, j) for i in timesteps for j in timeseries], name = 'var_1 ')

m.add_constraints(var_1[-1, j] == var_1[0, j] for j in series)


But this time, I run into the error:

File "...\docplex\mp\model.py", line 4190, in add_constraints
return self._lfactory._new_constraint_block1(cts)

File "...\docplex\mp\mfactory.py", line 972, in _new_constraint_block1
ctseq = list(cts)

File "...\main.py", line 324, in <genexpr>
m.add_constraints(var_1[-1, j] == var_1[0, j] for j in series)

KeyError: (-1, 0)


Could anyone offer any advice? All the best!

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