Jquery assumes that options.easing is either valid or evaluates to false. This is incorrect.

[_Nate_]

At the line:

this.pos = jQuery.easing[this.options.easing (jQuery.easing.swing ? "swing" : "linear")](this.state, n, 0, 1, this.options.duration);

options.easing is sometimes not undefined, 'linear', or 'swing', which results in a key error.

A fix is to put the lines

if(this.options.easing != 'swing' && this.options.easing != 'linear') this.options.easing = undefined

or something to that effect above the line mentioned above. However, there may be an underlying issue as to why this.options.easing is not undefined when not set