I guess tht's all one could say about the error
without looking code.

well, if ur using a collection class and in one of ur
iteration, you are not having any string value which
represents a number.

to debug
--------

1. Try printing the value of the variable.
2. If u get an empty string, then u might consider
adding a try-catch block as Jim suggested.
3. Parsing a non numeric string value will throw such
this error.