Null values in a DataColumn.PrimaryKey column

I am working with a DataTable.PrimaryKey field. I have two fields,
Address1 and Address2. The underlying database field for Address2 can
contain null values, and this is causing me problems - I get a "Column
'ADDRESS2' has null values in it" error. Is it possible to add a column
that may (and probably will) have nulls in the PrimaryKey? I have a
workaround in mind, but I just want to be sure before I try it.