"Other amount" option on donation pages is accepted with empty value

I created a contribution page as follows (using 3.1.3):

• Enable 'Execute real-time monetary transactions'
• No payment provider
• Pay later enabled
• Allow other amounts enabled
• Set a Minimum Amount of (for example) 1 euro.

Users can submit the contribution form checking the 'Other Amount' option, while leaving the 'Other amount' field empty. Amounts smaller than the set minimum receive a warning, but without any value entered, user can complete the transaction. The (missing) contribution amount is just not shown on the confirmation page.

Confirmed on the demo. Forum post here: http://forum.civicrm.org/index.php/topic,12484.0.html.