error: generate system identifier for general entity "sensor"

Validating http://www.huurjebus.nl

Error:

  1..  Line 23, Column 121: cannot generate system identifier for general
entity "sensor"
…/maps/api/js?key=AIzaSyD5WcktVE2PcIjKihGGhpcXcuxhLCS8xF4&sensor=true"></script>

Anyone can help me?

regards

Jaap


-- 
<http://www.conceptnoord.nl>