Merging a sublcass Structured Config that adds a field fails

[omry]

from dataclasses import dataclass
from omegaconf import OmegaConf, MISSING

@dataclass
class DBConfig:
    host: str = "localhost"
    port: int = MISSING
    driver: str = MISSING

@dataclass
class PostGreSQLConfig(DBConfig):
    driver: str = "postgresql"
    port: int = 5432
    timeout: int = 10

assert OmegaConf.merge(DBConfig, PostGreSQLConfig) == PostGreSQLConfig

Result:

omegaconf.errors.ConfigKeyError: Key 'timeout' not in 'DBConfig'
        full_key: timeout
        reference_type=Optional[DBConfig]
        object_type=DBConfig