quoting a bound pattern variable fails

Trying to quote an already bound pattern variable:

macro m {
    case $x => {
        macro n {
            case $[$x] => $x
        }
    }
}
m bar
n bar

gives the following error:

/Users/tim/Dropbox/dev/sweet-js/lib/expander.js:903
            if (head.hasPrototype(TermTree)) {
                     ^
TypeError: Cannot call method 'hasPrototype' of undefined
    at step (/Users/tim/Dropbox/dev/sweet-js/lib/expander.js:903:22)
    at step (/Users/tim/Dropbox/dev/sweet-js/lib/expander.js:1021:28)
    at enforest (/Users/tim/Dropbox/dev/sweet-js/lib/expander.js:1102:16)
    at expandToTermTree (/Users/tim/Dropbox/dev/sweet-js/lib/expander.js:1646:17)
    at expandToTermTree (/Users/tim/Dropbox/dev/sweet-js/lib/expander.js:1656:20)
    at expandToTermTree (/Users/tim/Dropbox/dev/sweet-js/lib/expander.js:1656:20)
    at expand (/Users/tim/Dropbox/dev/sweet-js/lib/expander.js:1795:21)
    at expandTermTreeToFinal (/Users/tim/Dropbox/dev/sweet-js/lib/expander.js:1677:43)
    at syntaxFromToken.value (/Users/tim/Dropbox/dev/sweet-js/lib/expander.js:1797:20)
    at Array.map (native)