Date-time as null

Hi,

Can't we keep date-time field of object type as null in swagger file? 

I am getting error in swagger parser that default=null is not of type 'date-time'.

please refer to below example

"voyageStartTime": {
"type": "string",
"format": "date-time",
"default": null,
"nullable": true
},
"voyageStopTime": {
"type": "string",
"format": "date-time",
"default": null,
"nullable": true
},