Cart, it looks like this same issue has happened with the bitcoin source.
Please see this link for how it was solved there:
https://github.com/bitcoin/bitcoin/issues/1969 .
Looks like you need to do a re-download of the blockchain.
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Diapolo commented on Oct 30, 2012
It seems that your blockindex got corrupted somehow. You should make a backup of your wallet.dat and then clean your data-directory and only leave in the wallet.dat. This leads to a re-download of the block-chain, which can take a while though.