如何为IntegerField设置NULL，而不是设置0?

I'm uploading some data from an excel file using xlrd and turning that data into models (with mainly IntegerField values) in Django. My excel file has a bunch of missing data. Unfortunately, these missing data are converted to the value of 0 in my models, instead of NULL. This means that when my Person model doesn't have an age, that age gets recorded as 0, instead of as NULL.I'm uploading some data from an excel file usin