NetworkXError: node is not in the graph - error in connecting nodes & transferring metadata?

[MauriceHendrix]

I got the following error:
NetworkXError: The node ion_diffusion$Nai is not in the graph.

Model: Shannon2004 (cellml attached [Shannon2004.zip (https://github.com/ModellingWebLab/cellmlmanip/files/4504436/Shannon2004.zip) ,it's a different version from standard)

Code to produce:

OXMETA = 'https://chaste.comlab.ox.ac.uk/cellml/ns/oxford-metadata#'  # oxford metadata uri prefix
var = model.get_variable_by_ontology_term((OXMETA, 'cytosolic_sodium_concentration'))
self._model.get_equations_for([var])

gives the error:
NetworkXError: The node ion_diffusion$Nai is not in the graph.

The ion_diffusion$Nai has as assigned_to the value Na_buffer$Nai something seems to have gone wrong connecting variables and transferring metadata