Authenticator gives 'this question requires a response' error with field filled in.

We have an authenticator with two fields. The first is prefilled in the url. The second is filled in by the respondent and checked against the contact list.
It works fine if the second field is correct, but if the user enters and incorrect response, gets the standard message and then enters the correct value, they get the 'this question requires a response' message.
I'm thinking that it is somehow losing the prefill value, even though it is still present in the URL.
If the screen is refreshed, it works fine again.