Schema:

{
  "anyOf": [{
    "type" : "object",
    "properties": {
      "key1": {"type": "string", "default": "A"}
    },
    "required": ["key1"]
  }]
}

input-data:

{}

should result in:

{
  "key1": "A"
}

But it produces errors if default is used. This happens in case of "anyOf" and "oneOf". "allOf" works.
Tested with ajv version 3.4.0 on Node.js v4.2.6 on 64bit Linux.
Option useDefault: true is used