Expressions that yield variant data-type cannot be used to define calculated columns.

I'm not really sure, why this is throwing this? I'm grabbing the earliest time?

ERFTime = 
IF (
    COUNTROWS (
        FILTER (
            RELATEDTABLE ( 'Apparatus Responses' ),
            'Apparatus Responses'[cancelledDateTime] <> BLANK ()
        )
    ) = 0,
    DATEDIFF (
        CALCULATE (
            MIN ( 'Apparatus Responses'[alarmDateTime] ),
            FILTER (
                'Apparatus Responses',
                'Apparatus Responses'[incidentID] = 'Incident Metadata'[incidentID]
            )
        ),
        CALCULATE (
            MAX( 'Apparatus Responses'[cancelledDateTime] ),
            FILTER (
                'Apparatus Responses',
                'Apparatus Responses'[incidentID] = 'Incident Metadata'[incidentID]
            )
        ),
        SECOND
    ),
    FALSE ()
)