Field collapsing source filtering bug

DELETE test

#Create index
PUT test
{
  "mappings": {
    "test": {
      "properties": {
        "colour": {
          "type": "keyword"
        }
      }
    }
  }  
}

#Add document 1
POST test/test/1
{
  "title": "this is a test",
  "colour": "blue"
}

#Add document 2
POST test/test/2
{
  "title": "this is another test",
  "colour": "blue"
}

#Query with collapse on colour field, source filtering both on top level document and on inner hits of collapse
#Throws an exception on the inner hits source filtering
#"reason": "[inner_hits] _source doesn't support values of type: START_ARRAY"
GET test/_search
{
  "_source": "title", 
  "query": {
    "match_all": {}
  },
  "collapse": {
    "field": "colour",
    "inner_hits": {
      "name": "collapse",
      "_source": [
        "title"
      ]
    }
  }
}

#Query with collapse on colour field, source filtering both on top level document and on inner hits of collapse
#Throws an exception on the inner hits source filtering
#"reason": "[inner_hits] _source doesn't support values of type: VALUE_STRING"
GET test/_search
{
  "_source": "title", 
  "query": {
    "match_all": {}
  },
  "collapse": {
    "field": "colour",
    "inner_hits": {
      "name": "collapse",
      "_source": "title"
    }
  }
}

#Query with collapse on colour field, source filtering both on top level document and on inner hits of collapse
#This works
GET test/_search
{
  "_source": "title", 
  "query": {
    "match_all": {}
  },
  "collapse": {
    "field": "colour",
    "inner_hits": {
      "name": "collapse",
      "_source": {
        "includes": ["title"]
      }
    }
  }
}