[R] Get a result but an error message as well ?

[varin sacha]

Dear R-experts,

Here is my R code, I get a result but I also get an error message so I doubt I can trust the result I get. 
What is going wrong ? Many thanks.

########################################
a<-c(2,4,3,4,6,5,3,1,2,3,4,3,4,5,65)
b<-c(23,45,32,12,23,43,56,44,33,11,12,54,23,34,54)
d<-c(9,4,5,3,2,1,3,4,5,6,4,9,10,11,18)

my.experiment <- function( ) {

OLS <- lm( a ~ b+d )
MSE_OLS<-mean(OLS$residuals^2)
return( c(MSE_OLS) )
}

my.data = t(replicate( 500, my.experiment() ))
colnames(my.data) <- c("MSE_OLS")
mean(my.data)
########################################