"Invalid Operation" when adding Oracle subscriber to SQL Server 2016 replication publication

Hi,

I'm trying to set up transactional replication from SQL Server 2016 to Oracle 12c.  (I realize that heterogeneous replication is deprecated but this should be for a limited time period).  I am following the general guidance here:https://msdn.microsoft.com/en-us/library/aa337389.aspx  and the Oracle-specific info here: https://msdn.microsoft.com/en-us/library/ms151864.aspx.  I am finding that when I try to add the non-SQL Server subscriber, I get this error:

Invalid operation. The connection is closed. (System.Data)

------------------------------
Program Location:

   at System.Data.SqlClient.SqlConnection.GetOpenConnection()
   at System.Data.SqlClient.SqlConnection.get_ServerVersion()
   at Microsoft.SqlServer.Management.UI.ReplicationSqlConnection.GetServerVersion()
   at Microsoft.SqlServer.Management.UI.ReplicationSqlConnection.get_IsServer130OrLater()
   at Microsoft.SqlServer.Management.UI.SubWizardSubscriber..ctor(String name, SubscriberType subscriberType, Boolean isSelected, Boolean isRegistered, ReplicationSqlConnection sqlConn, WizardInputs inputs)
   at Microsoft.SqlServer.Management.UI.PageChooseSubscribers.AddNonSqlSubscriber_OnClick(Object sender, EventArgs e)

I have tried to follow the Oracle/heterogenous recommendations, including

1) Installing recent 64-bit Oracle client and verifying basic connectivity to database

2) Ensuring the the distribution account has read access to the Oracle OLEDB provider directory

3) Setting public snapshot format to Character

4) Setting publication's subscription options to Allow non-SQL Server subscribers

5) Configuring my source SQL Server as distributor

Any other suggestions?  Any known guides or walk-throughs for this scenario?

thanks!

Martin