Any clues as to why I am receiving the following message?

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Invalid Serial Number.

The serial number entered does not appear to correspond to the product selected.
Please make sure you have selected the correct product from the drop down list.
Please check the serial number entered.Pay close attention to the letters and numbers, for they often look similar. For example:

* the number "2" and the letter "Z"
* the number "6" and the letter "G"
* the number "5" and the letter "S"
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I was able to register the product no problem, however when I went to request service on the iPod, I was unable to as I am receiving the following message constantly.

Would anyone please be able to help me regarding the issue?

Thanks in advance.