i don't get it, why did i get -2 rep for this?
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Someone downvoted your answer because they thought it was not helpful or incorrect. Downvoting incurs a 2 point penalty to the poster and a 1 point penalty to the voter.
Your answer was probably downvoted because it simply says "do this" without providing any explanation or context. Even if it did help the original author solve his problem (as evidenced by the accept), it is really not going to be very helpful to the typical Google visitor.
In the future, I suggest putting a little more detail into your answers - i.e., explain where that exception is to be thrown and why. Your comment reply (on your own answer) explains some of this; that information should have gone in the answer itself.