How do you treat missing value (n/a) cases of "citizenship status" in census 2000 and ACS up to 2013?

Would it be O.K. to treat N/A on “Citizenship Status” as undocuemented alien? If not, how do we estimate the undocuemented alien?

For the CITIZEN variable, persons with a value of “N/A” were born in the United States. Anyone born within the United States is a citizen; thus, persons with an “N/A” value should be treated as U.S. citizens by birthright. All other values of CITIZEN denote whether a foreign-born respondent acquired citizenship through other means (CITIZEN=1 or CITIZEN=2) or is not a citizen (CITIZEN=3). While undocumented immigrants would be included in the “Not a citizen” group, a variable does not exist to distinguish between undocumented immigrants and other non-citizens.

Hope this helps.