The issue is about the following lines in construct_object method of BaseConstructor class

I wanted to set up a multiconstructor for unknows tags. It seems like these lines are responsible for picking a default multiconstructor and therefore to define one I had to add a multiconstructor for None value as a tag_prefix

At the very least the lines hint that None is considered a valid key for self.yaml_multi_constructors. But the only way the execution can reach these lines is by traversing all the keys in self.yaml_multi_constructors first.

Now, if None is one of the keys, we get an exception TypeError: startswith first arg must be str, unicode, or tuple, not NoneType on line 77 making lines 83-84 effectively unreachable

Minimal non-working example:

import yaml

class MyLoader(yaml.loader.Loader):
    pass

def default_multi_constructor(loader, tag_suffix, node):
    return node.tag

MyLoader.add_multi_constructor(None, default_multi_constructor)

if __name__ == '__main__':
    print yaml.load('''!som:multitag {}''', MyLoader)