AX2009 - Fixed Asset Depreciation book

Hi there, in our environment, I have configured Value model to capture GL related entries and Depreciation book for Tax reporting purpose. When I created a new asset, and entered the details for the Depreciation book, I tried to change the depreciation convention (in the depreciation book) to “none”, I got the following error message

Cannot edit depreciation convention if Depreciation, Depreciation adjustment, Bonus depreciation, Extraordinary depreciation journals or transactions exist.

This is a new asset, still with status of “Not yet acquired”, can someone tell me why ??