Current Playback API - progress_ms: integer or string?

[MIGreenberg]

The web API supports getting information about the user's current playback state. The docs for this endpoint  detail the "progress_ms". This property contains the "Progress into the currently playing track" and is specified to be an integer. However, in the example JSON the property's value is of type string:

{
  "progress_ms": "44272"
}

Is this example incorrect, is the documentation incorrect, or something else? I know that some other APIs use strings to represent large numbers because of Javascript limitations.