Empty URLs Entity for some tweets


I have come across a few tweets that have a URL mentioned in the text but the URL entity is empty. See below:

    "created_at": "Thu May 05 11:47:03 +0000 2011",
    "id": 66106631431262208,
    "id_str": "66106631431262208",
    "text": "Chic French brand, The Kooples, is now open on First Floor in The Village. See their look book here http://on.fb.me/jSjmvd",
    "source": "<a href=\"https://about.twitter.com/products/tweetdeck\" rel=\"nofollow\">TweetDeck</a>",
    "truncated": false,
    "entities": {
        "hashtags": [],
        "symbols": [],
        "urls": [],
        "user_mentions": []
    "favorited": false,
    "retweeted": false,
    "lang": "en"

Under what circumstances is this entity populated?