为什么这会给我一个不确定的错误？

I am writing a simple csv file parser in go and cannot find out why I get "undefined: csvfile" and "undefined: err" with the following code. From all of the examples it appears to be correct.

var source string
flag.StringVar(&source, "file", "test.csv", "the file to parse")
flag.Parse()
csvfile, err = os.Open(source)

Use :=, not =, to create new variables:

csvfile, err := os.Open(source)