A deadlock was detected while trying to lock variables

I put the vbscript in event handler "OnError" for one of my packages (it has sequence container which contain multiple data flow tasks). If any of data flow tasks fail, I want to capture the error message and append to the user variable user::emailText.  However, I received error when I ran it.

   Description: A deadlock was detected while trying to lock variables "System::ErrorDescription" for read access and variables "User::emailText" for read/write access. A lock cannot be acquired after 16 attempts. The locks timed out.

Here is what I do in vbscript.

On property,

set ReadonlyVariables:System::ErrorDescription

set ReadWriteVariables: User::emailText

Dts.Variables("emailText").Value = Dts.Variables("emailText").Value.ToString + Dts.Variables("System::ErrorDescription").Value + vbCrLf