alignValue.toFixed is not a function

alignValue.toFixed is not a function

Hi,

I am using jQuery UI 1.8.11 in relation with jQuery 1.5.11.
I have a div-control which is transformed into a slider using the corresponding slider() function.
When sliding this control, the Firefox Javascript Console announces an error on line 427 in my (minimized) jQuery UI source file: alignValue.toFixed is not a function.
Sometimes, the slider does not work after this error, sometimes the slider works although. This is not deterministic.

From my point of view, it seems that this is a bug in the jQuery UI framework.
If required, I can give you more details.
Can anybody provide some help?

Cheers from Karlsruhe