can you provide some more information?
What interface are you using?
I donâ€™t think this will happen when creating the solver, but in some interaction.
A minimal example to reproduce this error would be ideal.

I found the error. It is because I wanted to set z with an empty array. Somehow this is not allowed. It may not make sense to set the z value, however I wanted to write a more general code, thus it was also included. It would be nice if this would not cause an error or at least raise a more meaningful exception.

Here is a minimal example:

import casadi as cd
import numpy as np
from acados_template import AcadosModel, AcadosOcp, AcadosOcpSolver
def create_model() -> AcadosModel:
model_name = 'minimal_example'
# set up states & controls
x = cd.SX.sym('x')
u = cd.SX.sym('u')
# xdot
x_dot = cd.SX.sym('x_dot')
f_expl = (x - u)**2
f_impl = x_dot - f_expl
model = AcadosModel()
model.f_impl_expr = f_impl
model.f_expl_expr = f_expl
model.x = x
model.xdot = x_dot
model.u = u
model.name = model_name
return model
def test_minimal_mpc_example_with_erk() -> None:
ocp = AcadosOcp()
model = create_model()
ocp.model = model
Tf = 2
N = 2
# set dimensions
ocp.dims.N = N
ocp.cost.cost_type = 'EXTERNAL'
ocp.cost.cost_type_e = 'EXTERNAL'
ocp.model.cost_expr_ext_cost = model.x**2
ocp.model.cost_expr_ext_cost_e = model.x**2
# set constraints
ocp.constraints.x0 = np.array([0])
# set options
ocp.solver_options.integrator_type = 'ERK'
ocp.solver_options.nlp_solver_type = 'SQP'
# set prediction horizon
ocp.solver_options.tf = Tf
acados_solver = AcadosOcpSolver(ocp, json_file='acados_ocp.json')
z_init = np.array([])
n_horizon = acados_solver.acados_ocp.dims.N
assert n_horizon
# When this for loop is commented out, the code works.
for stage in range(0, n_horizon):
acados_solver.set(stage, 'z', z_init)
acados_solver.solve()
if __name__ == '__main__':
test_minimal_mpc_example_with_erk()