script to add parent directory to source field

hello, i have a script i'm working on and i'm stuck here. 

i have a var that captures the source folder i'm working out of here, and the username of the computer working

var sourceFolderName = doc.path.name;

var userName = $.getenv('USER');

in part of the script i put the username into a metadata field here, and place the source folder into the keywords section of metada, but i'd rather it be in the "Source" field. 

doc.info.credit = userName;
var keywords = doc.info.keywords;
keywords.push(sourceFolderName);
doc.info.keywords = keywords;

i started to name the new rows to this

var source = doc.info.source;
source.push(sourceFolderName);
doc.info.source = source;

but that results in an error "ReferenceError: source.push is not a function: on line 88"