why is field_value not working when I try modifying it?

I want to extract N° from an invoice number. the field value is N° 68614 as you can see. I need just the number.

I tried differents methods, but no methods are working.

1.-

2.- 

3.-  

Finally I tried a simple method lower()

4.- 

I have installed python 3.3.2 and windows 7 on my computer.