Characteristics Error

When I create one characteristic like color, it correctly displays a drop-down window for the colors on the product page.
If I add a second characteristic like size, I get this error:
"Size No values found"

Am I doing something wrong?
How can I fix this so that I can display multiple characteristics?

If you have troubles with the display of variants of a product on the front end, you should:
1. Delete all the variants of the product on the listing of variants that you can access via the button "manage variants" on the product edition screen.
2. Delete the characteristics of the product.
3. Save the product.
4. Add the characteristics again.
5. The system will regenerate all the variants automatically and you should be able to see the dropdowns properly on the front end.

Thanks very much Nicolas.