Why the workflow end step cannot be the end of split node

[EveT0n]

I have a workflow with two steps before the end step

from metaflow import FlowSpec, step

class LinearFlow(FlowSpec):

    @step
    def start(self):
        self.my_var = 'hello world'
        self.next(self.a, self.b)

    @step
    def a(self):
        print('the data artifact is: %s' % self.my_var)
        self.next(self.end)

    @step
    def b(self):
        print('test b')
        self.next(self.end)

    @step
    def end(self):
        print('the data artifact is still: %s' % self.my_var)

if __name__ == '__main__':
    LinearFlow()

When I use it to create a workflow, the following error occurs

Validating your flow...
    Validity checker found an issue:
    Step end reached before a split started at step(s) a, b were joined. Add a join step before end.

I think the end step should be able to support multiple upstream steps. I read the documentation of metaflow and did not find an explanation of this design. My workflow runs on argo-workflow. Are there any other considerations for this limitation?