tapply and NA value

Hi,
I'm writing for a little help.
I have a dataframe with same NA value and I'd like to obtain the means of the 
value of a coloumn grouped by the levels of a factor coloumn of the datframe.
I'm using the function "tapply" but I see that if only a NA value is present 
the result is NA.
There is an option to have the correct result or I must use an other function?

Thanks of all
Leonardo

Leonardo Lami
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