Can't isolate an object with a DateTime field

[jasonfi]

Here's my test:
    
    
    import times
    import std/isolation
    
    type
      Test* = object
        d*: DateTime
    
    # Main
    var test = Test()
    let isoTest = isolate(test)
    
    
    Run

Compile fails with Nim v1.6.6: `Error: expression cannot be isolated: 
notificationMsg`

Other field types work fine, but I can't isolate an object with a DateTime 
field. Is this a known limitation?