Reporting use of <u></u> as invalid for XHTL Strict

Reporing an odd occurence in the W3 Validator.
Testing a document for XHTML(1.0) Strict validity.
 
I get an error such as this...
 



element "u" undefined.…sample of <strong>Bold</strong>, <u>Underline</u> and <em>Italic</em> text.I was pretty certain that was correct, and did quickly check things out.... <u></u> is apparently permissible and considered as valid XHTML Strict markup.Why is it telling me that the use of such is invlaid?Have I missed something?
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