Integration Services :: SSIS OnVariableValueChanged Deadlock

I'm having trouble getting the OnVariableValueChanged event handler to do anything useful. I've managed to get it to deadlock, but nothing else.

I have a simple package, with one "Execute SQL Task" which executes "SELECT 1 AS RC" and assigns the value to an SSIS variable "RowCount". I've set RaiseChangeEvent to true.I've also created an event handler at the package level for the OnVariableValueChanged event. In the eventhandler I have a Script Task, I select the variable "RowCount" as a readonly variable and in the script body I have...

public void Main()
{
// TODO: Add your code here
            String s = Dts.Variables["User::RowCount"].Value.ToString();
Dts.TaskResult = (int)ScriptResults.Success;
}

Very simple stuff. When I execute the package I get ...

Error: 0xC001405B at Script Task: A deadlock was detected while trying to lock variable "User::RowCount" for read access. A lock could not be acquired after 16 attempts and timed out.
Error: 0x2 at Script Task: A deadlock was detected while trying to lock variable "User::RowCount" for read access. A lock could not be acquired after 16 attempts and timed out.
Task failed: Script Task