R help - Apr 2006 - NA in dummy regression coefficients

I'm running a regression model with dummy variables and getting NA 
for some coefficients. I believe this due to singularity problem. How can I
exclude some of the dummy variables from the regression model in R to take care
of this issue. I read in R help that lm() method takes care of this issue
automatically. But in my case its not happening? Any pointers would be of great
help.
   
  Regression Model: 
   
  reg06 <- lm(mydf$y~ mydf$x1 + factor(mydf$x2) + factor(mydf$x3)+
factor(mydf$x4) +  mydf$x5, singular.ok = TRUE)
   
  Thanx in advance
   
  Sachin

		
---------------------------------

	[[alternative HTML version deleted]]