I am trying to validate a schema which should have a required key but may either have a populated array (of objects!) OR an empty array. (Using 2.6.0)

Here's my simple repro case. Schema first. Note that my array, if it had elements, would be an array of objects and not simple elements.

{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "type": "object",
  "properties": {
    "x": {
      "type": "array",
      "items": [
        {
          "type": "object",
          "properties": {
            "a": {
              "type": "string"
            }
          },
          "required": ["a"],
          "additionalProperties": false
        }
      ]
    }  
  },
  "required": [ "x" ],
  "additionalProperties": false
}

The populated case succeeds: {"x": [ {"a":"foo"} ] }

However, the empty array case of {"x": [] }' fails with
JSON::Schema::ValidationError: The property '#/x/0' of type NilClass did not match the following type: object in schema file:///zzz/schemas/schema.json
from /zzz/vendor/bundle/ruby/2.3.0/gems/json-schema-2.6.0/lib/json-schema/attribute.rb:18:in `validation_error'

Am I making a fundamental error with my schema, or is this a bug? I'm new at this but the way I read the spec, the empty array should be allowed in this case...