Error when adding an Invariant

[Sowmy Srinivasan <sow...@xxxxxxxxx>]

I am new to TLA Plus. I am writing a model for some replication stuff I am working on. When I add an Invariant property to the model checker I get the following error.
Invariant 'No value in valueList [] for 0. Bug in TLCModelLaunchDataProvider.' is violated by the initial state:

Console has the following:
@!@!@STARTMSG 2107:1 @!@!@
Invariant inv_14431923173141960000 is violated by the initial state:


Can someone throw light on what this error means?
Thanks
Sowmy
PS: Is TLA tool source available as open source?