[Bug]: Initial concentration function fails for DFN model

[d-cogswell]

PyBaMM Version

25.1.1

Python Version

3.12.4

Describe the bug

Passing an initial concentration function which makes use of r or x fails for DFN models. The same function runs without error for other models such as BasicDFN.

Steps to Reproduce

import pybamm


# x-dependent initial concentration
def initial_concentration(r, x):
    return 17038.0 + r + x


parameter_values = pybamm.ParameterValues("Chen2020")
parameter_values.update(
    {
        "Initial concentration in negative electrode [mol.m-3]": initial_concentration,
    }
)

model = pybamm.lithium_ion.DFN()
experiment = pybamm.Experiment(["Rest for 1 sec"])
sim = pybamm.Simulation(
    model,
    experiment=experiment,
    parameter_values=parameter_values,
)
solution = sim.solve()

Relevant log output

NotImplementedError: method self.evaluate() not implemented for symbol r_n of type <class 'pybamm.expression_tree.independent_variable.SpatialVariable'