DateTime NullReferenceException

[C# Programming]

In the code snippet below when i try to check if column row is not equal to null is throw an error: System.NullReferenceException: Object reference not set to an instance of an object

SPList srcList = web.List["My List Name"];
foreach(SPListItem item in srcList.Items)
{
if((DateTime)item["Start Date"] != null)
{
//do something
}
}