Multithreading error

[Ab]

get data from other server whith parallel.ForEach, i have an error

code:
Parallel.ForEach(list, async item =>{
var temp= await getdata(item);
mapper.Put(temp, "sheet_"+item, false);
})

error：NPOI.POIXMLException

NullReferenceException: Object reference not set to an instance of an object.

[Donny Tian]

sounds like an NPOI exception, I am not sure if NPOI supports multi-thread writing.