xml - Set exchange body to null -

i trying set null body of exchange in xml definiton this:

<camel:setbody>     <camel:constant>null</camel:constant> </camel:setbody> 

or this:

<camel:setbody>     <camel:simple>null</camel:simple> </camel:setbody> 

they give string "null" in end.

any idea right form is?

how this?

<camel:setbody>     <camel:simple>${bodyas(null)}</camel:simple> </camel:setbody>