strange warning when using hadd

Hello,
when trying to add root files using hadd I got the following warning message:

Warning in <TStreamerInfo::BuildCheck>:

        The StreamerInfo of class TArrayF read from file DTTimeBoxes_first_43553_0.root
        has the same version (=1) as the active class but a different checksum.
        You should update the version to ClassDef(TArrayF,2).
        Do not try to write objects with the current class definition,
        the files will not be readable.

Any idea of what the problem is?

thanks,

                    Marina