Unaccountable page title conflict when using single page copy API

Sometimes when using the single page copy API (/rest/api/content/{contentId}/copy) we randomly see this error:

  "statusCode" : 400,
  "data" : {
    "authorized" : false,
    "valid" : true,
    "errors" : [ ],
    "successful" : false
  "message" : "com.atlassian.confluence.api.service.exceptions.BadRequestException: Failed to save page while copying, unable to resolve page title conflict: 0"

“0” is not the title we specified and and to our knowledge there is also no page for that title in the space.

Has anyone else seen this? Is this a bug?