tapply confusion

Hello
I have a huge data frame with three columns 'Roof' 'Month' and 'Temp'
i want to run analyses on the numerical Temp data by the factors Roof and
Month, separately and together.
For using more than one factor i understand i should use aggregate, but i am
struggling with the tapply for single factor analysis.

 tapply(Temp, INDEX = Roof, FUN = median)

This works fine, however if i try to do anything a bit more complex, such
as:

tapply(Temp, INDEX = Roof, FUN = kruskal.test)

it gives the error - Error in length(g) : 'g' is missing

What could be the problem?
Thanks




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