tune(): how to specify a kernel ?

I would like to use tune() for tuning parameters for svm method, but it is 
not clear
to me how to specify a kernel.

I am trying to use something like this:

obj <- tune(svm, Species~., data = iris, ranges = list(kernel = 

c('radial', 'linear')))
Error in pmatch(x, table, duplicates.ok) :
        argument is not of mode character

but it generates an error.

Ryszard