You can get this error when trying to show a custom entity in a controller. Problem is probably your route. Make sure it has a "parameters" section pointing to the right entity type like:
contest.winner_detail:
path: '/contest/wall-of-fame/{id}'
defaults:
_entity_view: contest.winner
requirements:
_access: 'TRUE'
options:
no_cache: 'TRUE'
parameters:
id:
type: 'entity:contest'