Crash with FitContributor.createField on Fenix7

I have a crash when calling the following code on Fenix7 only:

            _estimated_hr_diff = createField(
                    "hrd",
                    1,
                    FitContributor.DATA_TYPE_FLOAT,
                    { :mesgType=>FitContributor.MESG_TYPE_RECORD, :units=>"bpm" }
                    );

Completely strange, this code works on a FR245 without any issues. Does anybody of you have issues with crating Fit data fields with a Fenix7?

The error itself fetched by ERA is completely odd: "Error Name: System Error"