error in DW when changing default extension.

document type "HTML" will not be added because it uses a file extension tha

[TinaBanana]

i tried to change my default extention from htm to html in dreamweaver and now i get an error when i launch.

i know i didnt it properly and have no errors.

document type "HTML" will not be added because it uses a file extension that is already associated with a prior Document Type

how can i fix this?

using DW mx 2004