Error : Class "date_api_argument_handler" not found

Hello,

I juste installed this module but I can't use it : when I try to add a date range when adding a new argument, an error occurred saying : "An error occurred at (...)?q=admin/build/views/ajax/add-item/frontpage/block_1/argument. Error description : Fatal Error : Class "date_api_argument_handler' not found in (...)/modules/modules/views_dateranger-6.x-1.0/views_daterange/daterange_argument_handler.inc on line 4"

What am I suppose to do ?

Thank you very much for your help ! :)