[R] Problem with plot()

[Andrej Kastrin]

Dear R useRs,

I have a problem to add title to the following graphics;

Tukey=TukeyHSD(aov(CA~C), "C",ordered=TRUE))
plot (Tukey, main="My first graph")

actually, it draw a graph, but it also display:

parameter "main" could not be set in high-level plot() function.

If I execute:
plot(rnorm(1000),main="My first plot"), no error occur.

What's the difference in those two examples? Thanks in advance.