fix: a second level of environment nesting expected a dict (#281)

Author: diefans

pydantic_settings/sources.py

@@ -620,7 +620,8 @@ def explode_env_vars(self, field_name: str, field: FieldInfo, env_vars: Mapping[
             target_field: FieldInfo | None = field
             for key in keys:
                 target_field = self.next_field(target_field, key)
-                env_var = env_var.setdefault(key, {})
+                if isinstance(env_var, dict):
+                    env_var = env_var.setdefault(key, {})
 
             # get proper field with last_key
             target_field = self.next_field(target_field, last_key)

tests/test_settings.py

@@ -2602,5 +2602,9 @@ class Settings(BaseSettings):
         'nested__foo__bar',
         'this should not be evaluated, since foo is a string by annotation and not a dict',
     )
+    env.set(
+        'nested__foo__bar__baz',
+        'one more',
+    )
     s = Settings()
     assert s.model_dump() == {'nested': {'foo': 'string'}}