Equal currency pairs don't work for some currencies after updating to Numbers 5.1

Since updating to Numbers 5.1 I have the following problem.

I use the CURRENCY function to get the current exchange rates between two currencies and do the conversion between them. For example, for the conversion between the EUR and USD I use:

=CURRENCY("EUR","USD","price").

Because of the way my spreadsheet is designed, sometimes it occurs to have the same currency in the same function, returning the value 1.00. For example, CURRENCY("EUR","EUR","price") returns 1.

However, for some reason the same doesn't work for many other currencies such as "SEK".

This is best illustrated on the following example: I have 4 currencies in a table, obtaining 16 mutual rates from their combinations:

Pairs "EUR"<>"EUR" and "USD"<>"USD" normally return 1, but the same doesn't apply to "SEK" and "PLN". I get the following error:

As visible from the screenshots, both currencies SEK and PLN work when paired with other currencies.

Is there any particular reason why same currency pairs don't fully work after the update to 5.1, and why it works for some currencies, while for others it returns an error?