Changing a DataFrame entry using "chop"

A column in my DataFrame contains Strings that must be cleaned. I was hoping the following method would work, but it does not. What would work?

df[i,3] = chop(df1[i,3], head = 10, tail = 0)

I get the error “setindex! not defined for CSV.Column2{String,PooledString}

My guess is you aren’t working with a DataFrame. do df = DataFrame(df) first.