Get subject of selected messages in Mail

The following script

tell application “Mail”
activate
set a to selected messages of message viewer 1
end tell

returns something like

{message 2 of mailbox “SWAP” of application “Mail”}

but if I change it to

tell application “Mail”
activate
set a to subject of selected messages of message viewer 1
end tell

it errors

tell application "Mail"
	--activate--<<<<-optional
	set a to selected messages of message viewer 1
--returns a list of message(s)
	set b to subject of (item 1 of a)--<--< 
end tell