How is my mitems iterator wrong?

[ggibson]

I'm practicing wrapping std::vector before attempting trickier things. I have 
the items iterator working, but the compiler complains I need an initializer 
when using mitems. All I can figure is that I'm missing wrapping some 
std::vector function related to initialization. I'd be grateful if someone 
could explain what's going on here.
    
    
    {.pragma:vecheader, header:"<vector>".}
    type vector {.importcpp:"std::vector", vecheader.}[T] = object
    proc newVector[T]():vector[T] {.importcpp:"std::vector<'*0>()", vecheader.}
    proc newVector[T](size:int):vector[T] {.importcpp:"std::vector<'*0>(#)", 
vecheader.}
    proc newVector[T](size:int, val:T):vector[T] 
{.importcpp:"std::vector<'*0>(@)", vecheader.}
    proc `[]`[T](base:var vector[T], idx:int):var T {.importcpp:"#[#]", 
vecheader.}
    proc `[]=`[T](base:var vector[T], idx:int, val:T) {.importcpp:"#[#]=#", 
vecheader.}
    proc push_back[T](base:var vector[T], val:T){.importcpp:"#.push_back(@)", 
vecheader.}
    proc size[T](base:vector[T]):int {.importcpp:"#.size()", vecheader.}
    iterator items[T](v:var vector[T]): T =
       for i in 0..<size(v):
          yield v[i]
    iterator mitems[T](v:var vector[T]):var T =
       for i in 0..size(v):
          yield v[i]
    
    var vec = newVector[string](3)
    for i in 1..size(vec):
       vec[i-1]="astring"
    for item in vec.mitems:
       echo item