Note to self…

When you see this Application event log entry:

Event Type:         Warning 
Event Source:       BizTalk Server 2009 
Event Category:     (1) 
Event ID:           5743 
Date:               6/26/2009 
Time:               8:06:42 PM 
User:               N/A 
Computer:          CSD-Labs 
The adapter failed to transmit message going to send port "SAPPortBilling" with URL "sap://CLIENT=999;LANG=EN;@a/". It will be retransmitted after the retry interval specified for this Send Port. Details:"System.ArgumentNullException: Value cannot be null. 
Parameter name: key 
   at System.ThrowHelper.ThrowArgumentNullException(ExceptionArgument argument) 
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add) 
   at Microsoft.BizTalk.Adapter.Wcf.Runtime.WcfClient`2.CreateChannelFactory[TChannel](IBaseMessage bizTalkMessage) 
   at Microsoft.BizTalk.Adapter.Wcf.Runtime.WcfClient`2.InitializeValues(IBaseMessage message) 
   at Microsoft.BizTalk.Adapter.Wcf.Runtime.WcfClient`2..ctor(IBaseMessage message, WcfTransmitter`2 transmitter) 
   at Microsoft.BizTalk.Adapter.Wcf.Runtime.WcfTransmitter`2.GetClientFromCache(String spid, IBaseMessage message) 
   at Microsoft.BizTalk.Adapter.Wcf.Runtime.WcfAsyncBatch`2.BatchWorker(List`1 messages)". 

Make sure you have entered something in the Action part when configuring BizTalk SAP Adapter Send Port.