Weird -1/0 exchange rates appear from time to time

[Paul Salmon]

I can't figure out what causes this, but these entries find their way
into my GnuCash file from time to time. Here's one that appeared
within the last 30 days:

  <price>
    <price:id type="guid">d8925eb5c06f2bfa4287f7b25b315605</price:id>
    <price:commodity>
      <cmdty:space>ISO4217</cmdty:space>
      <cmdty:id>USD</cmdty:id>
    </price:commodity>
    <price:currency>
      <cmdty:space>ISO4217</cmdty:space>
      <cmdty:id>UAH</cmdty:id>
    </price:currency>
    <price:time>
      <ts:date>2012-05-27 00:00:00 +0300</ts:date>
    </price:time>
    <price:source>user:xfer-dialog</price:source>
    <price:value>-1/0</price:value>
  </price>

Most of the time the ts:date is a recent one, so this one is extra weird.

I notice it when my file is processed by a separate tool, which
(understandably, IMO) fails when it encounters this exchange rate.

Any ideas on how to stop this happening? Using v2.6.5 on Win7.