Line 4723 of the 9/25 nightly build does a typeof check against the "speed" parameter:

4722     speed: function(speed, easing, fn) {
4723         var opt = typeof speed === "object" ? speed : {
4724             complete: fn || !fn && easing ||
4725                 jQuery.isFunction( speed ) && speed,
4726             duration: speed,
4727             easing: fn && easing || easing && !jQuery.isFunction(easing) && easing
4728         };  
4729             
4730         opt.duration = jQuery.fx.off ? 0 : typeof opt.duration === "number" ? opt.duration :
4731             jQuery.fx.speeds[opt.duration] || jQuery.fx.speeds._default;

If the "speed" parameter is null, the typeof expression evaluates to "object", causing "opt" to be set to null. This triggers an error in line 4730 below.

My fix is to add a "

speed &&

var opt = speed && typeof speed === "object" ? speed : {