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A non-zero value is calculated for a FixedRateBondForward, if the issue date of the underlying bond occurs on or after the bond's maturity.
This non-zero value persists regardless of the time difference between issue date and maturity.
Click on download to download an xml file that contains an Excel formula that demonstrates this issue.
You may then reproduce the issue in spreadsheet by clicking on Go -> Load Excel Formula from Text File or one of its variations and choose the xml file downloaded in the step above.