Out of Order in repeating group if first tag of repeating group is not set (even if it is not a mandatory field)

In quickfix.Message, parseGroup(...) the code looks for the first field in the group as defined by the dictionary. If it is not defined, then a "REPEATING_GROUP_FIELDS_OUT_OF_ORDER" exception is triggered.

I made a TradeCaptureReport message. This type of message has a "Symbol" field in the "Instrument" component.
I used the "NoLegs" repeating group and the message parsing on the server failed because I did not put the "LegSymbol" field in the group.

My workaround is to put the "LegSymbol" field for each group. It makes the message bigger with no added value.

If there is a reason to make this check, could somebody explain ?

Thanks,
John