Occurs on mysql & postgres

Prerequisites:
Create product type and enable some currencies

Steps to reproduce:
1. Create a new product
2. Go to the prices tab
3. Select currency and click on 'Create' button
4. Fill Base price box (at the top)
5. Fill Custom price box
6. Click Create button

(Also happens when editing product)

Actual result:
Error 500 occurs.

Expected result:
Validation notifications occurs, user is unable to proceed.