internal error: genTraverseProc()

[dom96]

type
  PFuture*[T] = ref object
    value*: T
    finished*: bool
    cb: proc (future: PFuture[T]) {.closure.}

var k = PFuture[void]()

a16.nim(3, 4) Error: internal error: genTraverseProc()
Traceback (most recent call last)
nimrod.nim(87)           nimrod
nimrod.nim(55)           HandleCmdLine
main.nim(308)            MainCommand
main.nim(73)             CommandCompileToC
modules.nim(193)         CompileProject
modules.nim(151)         compileModule
passes.nim(192)          processModule
passes.nim(136)          processTopLevelStmt
cgen.nim(1257)           myProcess
ccgstmts.nim(964)        genStmts
ccgexprs.nim(1886)       expr
ccgstmts.nim(964)        genStmts
ccgexprs.nim(1906)       expr
ccgstmts.nim(193)        genVarStmt
ccgstmts.nim(176)        genSingleVar
ccgstmts.nim(58)         loadInto
ccgexprs.nim(1866)       expr
ccgexprs.nim(1021)       genObjConstr
ccgexprs.nim(964)        rawGenNew
ccgtypes.nim(938)        genTypeInfo
ccgtrav.nim(122)         genTraverseProc
ccgtrav.nim(75)          genTraverseProc
ccgtrav.nim(29)          genTraverseProc
ccgtrav.nim(48)          genTraverseProc
msgs.nim(814)            InternalError
msgs.nim(791)            liMessage
msgs.nim(707)            handleError