[Typo3] Extension problem

[Roberts, Emma]

Hi, 
I've got a problem with my local install of Typo3 - I'm trying to test a few
extensions but when I try to upload them I get this error - 

Error: No extension key!!! Why? - nobody knows... (Or no files in the
file-array...)

They work fine when I upload them on the live server though so it doesn't
seem to be anything to do with the actual extension.   

Any ideas anyone?

Emma
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