This issue is typically caused by the account in question having a transaction with a NULL (empty) activity type value.  You should check the transactions for the account in question for missing activity type values.  This can be done by Zooming <F10> on the Source field in Giving History Detail.  If identified, then the transaction(s) can be adjusted to reflect the correct value. 

This issue is most frequently the result data missing during the initial conversion.  If the batch item in error is an adjustment, this can sometimes be resolved be deleting, then reentering the adjustment in a separate batch.

If you encounter this error and require assistance, please create a support request at providing the batch number and error log and referencing this solution.