freeze() does not work on collections.defaultdict

[connorjward]

The following code

import collections
from pyrsistent import freeze

mydict = collections.defaultdict(dict)
mydict["a"]["x"] = 2
mydict["a"]["y"] = 3
mydict["b"]["z"] = 4

frozen = freeze(mydict)
print(type(frozen))

prints <class 'collections.defaultdict'> rather than PMap as I would expect. In other words freeze does not do anything to a collections.defaultdict when I would expect it to make it a persistent map instead.