Pydantic enum field does not get converted to string

I am trying to restrict one field in a class to an enum. However, when I try to get a dictionary out of class, it doesn't get converted to string. Instead it retains the enum. I checked pydantic documentation, but couldn't find anything relevant to my problem.

This code is representative of what I actually need.

from enum import Enum
from pydantic import BaseModel

class S(str, Enum):
    am = 'am'
    pm = 'pm'

class K(BaseModel):
    k: S
    z: str

a = K(k='am', z='rrrr')
print(a.dict()) # {'k': <S.am: 'am'>, 'z': 'rrrr'}

I'm trying to get the .dict() method to return {'k': 'am', 'z': 'rrrr'}

from enum import Enum
from pydantic import BaseModel

class S(str, Enum):
    am='am'
    pm='pm'

class K(BaseModel):
    k:S
    z:str

    class Config:  
        use_enum_values = True  # <--

a = K(k='am', z='rrrr')
print(a.dict())