Topic: measureText (HTML5 canvas) not working

I've got this function in javascript that uses HTML5's canvas function measureText()

measureText: function(text) {
    this._ctx.font = '12px Arial';
    return this._ctx.measureText(text);
},

This should return a text metrics object works in all browsers except epic where
measureText is returning "false" which upsets the calling function.

Any clues?