That is what I thougth as well. According the manual you can use an expresion, column, constant or bind variable for N.
N determines the nth row for which the measure value is to be returned. n can be a
constant, bind variable, column, or an expression involving them, as long as it
resolves to a positive integer. The function returns NULL if the data source window
has fewer than n rows. If n is null, then the function returns an error.