Cannot assign points

[Bubba]

Take a look of the picture below:



As you see i've got a skill point still to use and i want to spend it on "defence". I just used 1 (my player gained 2 skill points) on defence but...look, it will not let me do it!!!
The botton is disabled (it's grey and not blue, like the others).
The player is not in auction...could you tell me what's the problem?
Thank you!