Can't use active_material.BaseModel submodel missing variable

[ch662]

Hello, I am running Pybamm 22.9 and I am trying to use this (active_material.BaseModel) submodel with DFN since I have an active material fraction that varies with position. However, I get the following error:

ModelError: Missing variable for submodel 'negative particle mechanics': 'Negative electrode surface area to volume ratio'.

import pybamm

modelMLA = pybamm.lithium_ion.DFN(build = False,options=options)

options = {
"lithium plating": "partially reversible",
"SEI on cracks":"true",
}

modelMLA.submodels["negative primary active material"] = pybamm.active_material.BaseModel(
modelMLA.param,"negative", options
)

modelMLA.build_model()`