The -windowtitle option requires an argument. It sounds like your javadoc command has the -windowtitle option with no argument.
http://java.sun.com/j2se/1.4.2/docs/tooldocs/windows/javadoc.html#windowtitle
If it's not clear, show us your command and we can figure it out.
-Doug
I too am generating my javadoc files from Netbeans and I get the same message. However, my javadoc is generated fine, so my advice is not to worry about it!
The documentation says:
-windowtitle
Specifies the title to be placed in the HTML <title> tag. This appears in the window title and in any browser bookmarks (favorite places) that someone creates for this page. This title should not contain any HTML tags, as the browser will not properly interpret them. Any internal quotation marks within title may have to be escaped. If -windowtitle is omitted, the Javadoc tool uses the value of -doctitle for this option.