Optional arguments for a custom InsetLayout?

I was trying to write a really small module for the sidenotes package. I 
pretty much copied what jürgen Spitzmüller wrote for the pdfcomment 
package (http://wiki.lyx.org/uploads/Modules/pdfcomment.module)

Now what I realized is, that I can't insert optional arguments (the LyX 
way, via insert>short title), even though I specified their availability 
in the module. The pdfcomment module also seems to enable optional 
arguments, but I can't add any when I try it.

Does anyone know what I am doing wrong or if this is a bug? The code for 
the module is below.

Thanks for your help!

#\DeclareLyXModule{Sidenotes}
#DescriptionBegin
#Inserts sidenotes in the output.
#DescriptionEnd
# Author: Florian Rubach

Format 11

InsetLayout sidenote
     LyXType               custom
     LabelString           "Sidenote"
     LatexType             command
     LatexName             sidenote
     Decoration            classic
     LabelFont
       Color               cyan
       Size                Small
     EndFont
     MultiPar              false
     OptionalArgs          2
     Preamble
         \RequirePackage{sidenotes}
     EndPreamble
End