Problem with variables for replacement

Hi,

I mapped 15 Fields of my content type.

In the parser config I want to use the variable of an above field. Everything works fine and everything is imported correctly. But there occurs some errors with this message:
Libxml returned the message: Invalid expression, with the error code: 1207.

The problem is the following: The first element was missing in my XML structure so that the field stays empty. Due to that the variable could not be used for replacement.

But everything works fine. All data was imported.

Is there an option to surpress the errors or to check the variable (in the background) before using it?