[xmonad] DynamicWorkspaces: New WS not accessible with mod+number

I prefer using only few workspaces by default and adding one if needed, whether by adding a workspace with addWorkspace and rename it or, more easily, by using selectWorkspace and entering a number that doesn't exist. In both cases, i can't access the new workspace by "mod + number of workspace". Is there a workaround? My xmonad has version 0.9-1. Ralph