LR4: error message on import : attempt to call field 'isNSImage' (a nil value)

I have just installed LR4 on my mac and when I click on import I get the error message -

: attempt to call field 'isNSImage' (a nil value)

Can anyone give me any idea what this means or how to rectify it.

I am new at all this and really don't have a clue

Thanks