ConstructorError - could not determine a constructor for custom tag (5.1)

The following code was working on 3.13 but no longer works in 5.1:

import yaml

class Ref(yaml.YAMLObject):
    yaml_tag = '!Ref'
    def __init__(self, val):
        self.val = val

    @classmethod
    def from_yaml(cls, loader, node):
        return cls(node.value)

yaml.load('Foo: !Ref bar')

I get the following exception on 5.1:

yaml.constructor.ConstructorError: could not determine a constructor for the tag '!Ref'
  in "<unicode string>", line 1, column 6:
    Foo: !Ref bar

This is true whether I use yaml.load, yaml.full_load or yaml.unsafe_load.

[perlpunk]

Try

yaml.load('Foo: !Ref bar', Loader=yaml.Loader)

I’m not sure though if this is intentional. Will have a closer look later.

[rmarou]

None of the workarounds above worked for me, I had to use this loader: Loader=yaml.BaseLoader on Python 3.7.0 PyYaml==5.1.2 or == 5.2b1

(edited) I reproduced using the same code: yaml.load('Foo: !Ref bar') This worked for me :yaml.load('Foo: !Ref bar', Loader=yaml.BaseLoader)