dynamic submission actions?

> <xforms:submission id="save-submission"
> ref="instance('manualScript-instance')" f:url-type="resource"
>
> action="/exist/rest/db/orbeon/score-script/{{xxforms:instance('manualScript-instance')/st:name}}"
> method="put" replace="none">
>
> I got this syntax in the action attribute from the sample apps.  I figured
> the {{ }} construct would allow an xpath expression, that I would use to
> return the name of the object I want to save.
>
> And that would save each object under a different name.  
>
> But it doesn't work, I get
> 2007-05-08 10:22:34,304 http-8080-2 DEBUG processor.XFormsServer null -
> XForms - opening URL connection for:
> http://localhost:8080/ops/exist/rest/db/orbeon/score-script/%7Bxxforms:instance('manualScript-instance')/st:name%7D
>
> and
>
> 2007-05-08 10:22:34,320 http-8080-2 ERROR processor.XFormsServer null -
> XForms - submission - xforms-submit-error throwable:
> org.orbeon.oxf.common.OXFException: Error code received when submitting
> instance: 500
> at
> org.orbeon.oxf.xforms.XFormsModelSubmission.performDefaultAction(XFormsModelSubmission.java:799)
> at
> org.orbeon.oxf.xforms.XFormsContainingDocument.dispatchEvent(XFormsContainingDocument.java:920)
> at
> org.orbeon.oxf.xforms.control.controls.XFormsSubmitControl.performDefaultAction(XFormsSubmitControl.java:48)
> ...
>
> in my logs.  The substitution doesn't seem to be happening, anyone know how
> to do this properly?
>
> THanks,
>
> Jeff
>