Cannot write the graph in any extension

[ahmedibatta]

I am working with neurommsing graph.
I loaded through their package and then union all graphs but cannot save

I got error : Invalid extension for file: neurommsing.bel

code

graphs = repository.get_graphs()

#Combine them all using pybel.union
graph = union(graphs.values())
pybel.dump(graph, 'neurommsing.bel')

error : Invalid extension for file: neurommsing.bel'

I checked

graph : <pybel.struct.graph.BELGraph at 0x7f379eafded0>