HTMLGEN: arguments can't be adjustable strings?

The following works:

CL-USER(667): (let ((zero (make-array 1
                             :element-type 'character)))
                (setf (aref zero 0) #\0)
		(net.html.generator:html-stream 
		 *standard-output*
		 ((:table :border zero))))
<table BORDER="0"></table>
"</table>"

But if I use an adjustable string instead, HTMLGEN generates an error:

CL-USER(668): (let ((zero (make-array 0
                             :element-type 'character
			     :adjustable t
                             :fill-pointer t)))
                (vector-push-extend #\0 zero)
		(net.html.generator:html-stream 
		 *standard-output*
		 ((:table :border zero))))
<table BORDER="
Error: `"0"' is not of the expected type `SIMPLE-STRING'
  [condition type: TYPE-ERROR]

Restart actions (select using :continue):
 0: Return to Top Level (an "abort" restart).
 1: Abort entirely from this process.
[1] CL-USER(669): 

A bug?

Andrew