How to compare default struct value (no null)? Why this code does not work?

Question

How to get the default value of a struct, why this code does not work?

KeyValuePair<DateTimeOffset, Alarm>  kvpAlarmDuplicate = lastestUniqueAlarms.FirstOrDefault(kvp => kvp.Value.OriginalUniqueMessage == originalUniqueMsg);
                    
if (kvpAlarmDuplicate == default(KeyValuePair<DateTimeOffset, Alarm>)) // Here error*
{
        ...
}
                    
Error* => CS0019 Operator '==' cannot be applied to operands of type 
  'KeyValuePair<DateTimeOffset, Alarm>' and
  'KeyValuePair<DateTimeOffset, Alarm>' 

How to know if the returned value is the default one?

Additional info (I don't know if it is useful, just in case)? =>

[Table("alarm")]
public class Alarm
{
    public Alarm()
    {
    }

    public static TimeSpan DelayDuplicateOccurence { get; set; } = TimeSpan.FromMinutes(10);

    public int AlarmId { get; set; } 
    ...