first on a stirng series gives error

I am having an error when using first on a string series, can someone let me know what is wrong with my code ?
I am playing with the series feature and see how it works. I am trying to select an option from a drop down list and then select a file from my c: drive . the option selected is "now" and the file name is "new.xls",
I pick the first character of "now" and compare with the first character of "new.xls" , so I expect both to yield a true if they are equal. which they are in this case.
However it's giving me an error message.

the code is below:-


rebol [ ]

view layout [
k: drop-down "Hello" "Now" "Bre" "You"

; there is a file new.xls in c/test folder
button "click me" [ z: request-file/file %/c/test

print series? (find/last form z "/"
print second (find/last form z "/"


print series? k/text
print first k/text

;if I choose "now" from the drop-down and new.xls from
;c/test folder, then if I compare the first character
;this should give me a true
if (first k/text = second (find/last form z "/")) [
print "true"
]

]

]


** script error: first expected series argument of type: series pair ....

** near: if first k/text = second

Thanks Grahmam for reviewing the code and the solution provided.