Error in read.csv with colClasses: scan() expected 'a real' got 'NULL'

I am reading in large csv file using `read.csv`. Several websites suggest using colClasses to define the classes for each column to make the import process faster. ``` t = read.csv("pca.csv",header=TRUE,colClasses = classes) Error in scan(file, what, nmax, sep, dec, quote, skip, nlines, na.strings, : scan() expected 'a real', got 'NULL' classes = c("numeric","integer") ``` I obviously have nulls in some of my data. Is there a way to use colClasses where "numeric" or "integer" include nulls? Also, any other tips on importing large datasets faster into R would be very helpful. I have all the data in a SQL database and I've tried using RODBC which is surprisingly slower than read.csv().