Can I increase my intelligence from a Headband Of Intellect?

2 sessions ago, my wizard found a headband of intellect, and used it to up his spells.

Your Intelligence score is 19 while you wear this headband. It has no effect on you if your Intelligence is already 19 or higher without it.

Now I am close to getting an ability score increase and I want to increase my intelligence. So my question is, can I increase his intelligence from 19 to 20, or do I have to go from my non-modified intelligence?