[R] Ancova doesn't return test statistics

[Johan A. Stenberg]

Dear all,

I'm quite sure that this is a stupid question, but I'll ask anyway.
I want to perform an ANCOVA with two continuous factors and three 
categorical factors.

Plant population growth rate (GR) = dependent variable
Seed reduction due to herbivory (SR) = continuous explanatory variable
Herbivore species (HS, 2 levels) = categorical explanatory variable
Population (Pop, 24 levels) = categorical explanatory variable
Population size (Popsize) = continuous explanatory variable
Year (Year, 16 levels) = categorical explanatory variable

My model is technically simple:

model<-aov(GR~SR*HS*Pop*Popsize*Year)

However, R is not returning any F and P values – only Df, Sum Sq and 
Mean Sq. I have to remove either Year or Pop in order to get the test 
statistics. Why is this?

Thank you in advance!
Johan A. Stenberg, Umea University, Sweden