Error when remove html tag when run in portlet mode

I'm running tapestry in portlet mode, so in the .tml template I want to remove the html tag. But I encounters this error: *The root element of the rendered document was , not . A root element of is needed when linking JavaScript and stylesheet resources.* Stack trace: - org.apache.tapestry5

Error when remove html tag when run in portlet mode

I'm running tapestry in portlet mode, so in the .tml template I want to remove the html tag. But I encounters this error: *The root element of the rendered document was , not . A root element of is needed when linking JavaScript and stylesheet resources.* Stack trace: - org.apache.tapestry5