I am not sure how you configured the value that is displayed in the exception message just by resizing the RadDock control, however I succeeded in reproducing the problem by editing the size of the dock directly in its client state. I have brought it to the attention of our developers, nevertheless I cannot provide a firm estimation when it will be fixed.
You can use the bug report item that I logged in order to follow the status of the bug, vote for it and comment it: http://feedback.telerik.com/Project/108/Feedback/Details/126013
I have updated your Telerik points as a small token of gratitude for reporting this issue.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.