unable to refine grouping where field in grouping was an extension

a YANG submodule has a statement of the form:

uses foo {
refine bar

}

this fails in parsing with an error along the lines of:

"Refine target node bar not found."

grouping foo is defined in the same module and only contains bar. However bar is defined using a vendor extension

grouping foo {
vendor:extension bar

}

I'm guessing ODL isn't happy with the vendor extension...