This open statement shadows the label

I have another one of these I don’t get:

module DOMAPI = {
  type timeoutId
}

module Global = {
  open DOMAPI

  external setTimeout: (~handler: unit => unit, ~timeout: int=?) => timeoutId =
    "setTimeout"
}

[W] Line 6, column 2:
this open statement shadows the type identifier timeoutId (which is later used)

What is wrong here? How is anything shadowed?