It's a bit difficult to say anything without the full error message and the code it relates to.
But what does the error say: you're trying to reference a form variable, that doesn't exist. I suspect this is because you're trying to reference a form variable, that doesn't exist. Just a guess.
The curious bit is that said form variable doesn't appear to have a name. I suspect you've got some code that acts on dynamically-named form fields, and the code has a logic error in it.
--
Adam
