What are the values of the Index and Item being passed to the function? Once you find out which one is causing the error, you need to trace that value back to see where it is getting set and why it's getting set to a non-valid value.
Also, according to amxmodx.inc, you should be using get_func_id() for getting the IDs to be using with callfunc_begin_i(). This is probably unrelated but it's better to use functions how they were intended.