How to specify null foreign key in symfony doctrine fixture

Here is a Doctrine schema:

Device:
  columns:
    name:           { type: string(60), }
    os_version_id:  { type: int(11), notnull: false }
  relations:
    OsVersion:
      foreignAlias: Devices

OsVersion:
  columns:
    version: { type: string(60), fixed: false, notnull: true }

Here is a fixture:

OSV_ABCD:
  version: ABCD

OSV_DEFG:
  version: DEFG

Device:
  Device_1:
    name: 'My device'
    OsVersion开发者_开发知识库: null

When loading this fixture I get this:

Invalid row key specified: (os_version) , referred to in (device) Device_1

If I change OsVersion: null to OsVersion: OSV_ABCD it loads without problem.

Omitting the foreign key row entirely worked:

Device:
  Device_1:
    name: 'My device'