Using XML as Input Parameter Reslts in "Invalid at the top level of the document"

In an Execute SQL Task i'm using a stored procedure with one input parameter. The parameter is defined as varchar(max), but the expectation is that valid xml would be passed in. The stored proc subsequetly opens the xml. When the xml string is relatively short, everything works fine. But for longer XML strings, the Execute SQL task fails. One of the messages is "Invalid at the top level of the document". When I capture the command via a trace and run it directly from SSMS, everything runs fine. There are more details, but I'm not sure what is important.