undefined map

[sidkdbl07]

I get an error stating that

TypeError: Cannot read property 'map' of undefined

var map = L.map('map').setView([51.505, -0.09], 13);

L.tileLayer('http://{s}.tile.osm.org/{z}/{x}/{y}.png').addTo(map);
L.marker([51.5, -0.09]).addTo(map)
  .bindPopup('A pretty CSS3 popup.<br> Easily customizable.')
  .openPopup();