What is going on here?

Code:

class Message
{
...
}

task.send(new immutable(Message)(args));

source/vibe/concurrency.d(1148): Error: can only initialize const
member expandfield_0 inside constructor
source/vibe/concurrency.d(1065): Error: template instance
vibe.core.concurrency.variantWithIsolatedSupport!(immutable(Message))
error instantiating
source/client/connection.d(54): instantiated from here:
send!(immutable(Message))
source/client/connection.d(54): Error: template instance
vibe.core.concurrency.send!(immutable(Message)) error instantiating