I get a strange error when doing: LatexTOC

Error detected while processing function LatexBox_TOC..<SNR>47_ReadTOC:
line 44:
E684: list index out of range: 1
E116: Invalid argumenst for function empty(tree[1][1])
E15: Invalid expression: !empty(tree[1][1])
line 54:
E121: Undefined variable: secnum
E116: Invalid arguments for function add
line 44:
E684: list index out of range: 1
E116: Invalid argumenst for function empty(tree[1][1])
E15: Invalid expression: !empty(tree[1][1])
line 54:
E121: Undefined variable: secnum
"LaTeX TOC" [New File]
Press ENTER or type command to continue

It's a XeLaTeX file - if that makes any difference