unexecutable code in chapter 1 about statistics-for-data-scientists HOT 2 OPEN

Has anyone had issues running the code on page 31?
I keep getting the following error message.
Error in x[j] : invalid subscript type 'list'

from statistics-for-data-scientists.

I'm getting a different issue with the code on this page.

Running sp500_sym[sp500_sym$sector=="etf", 'symbol'] yields the etfs you find on page 32 when running corrplot in the book. HOWEVER, running

etfs <- sp500_spx[row.names(sp500_px)>"2012-07-01", sp500_sym[sp500_sym$sector=="etf", 'symbol]] yields a different set of etfs.

i.e.
> sp500_sym[sp500_sym$sector=="etf", 'symbol']
> [1] XLI QQQ SPY DIA GLD VXX USO IWM XLE XLY XLU XLB XTL XLV XLP XLF XLK

but

> colnames(etfs)
[1] "VAR" "PBCT" "CINF" "ALLE" "JCI" "ENDP" "THC" "IPG" "SYK" "MCK" [11] "CERN" "STJ" "BXLT" "ESRX" "CAH" "TMO" "ZBH"

I admit that I am still learning R, so could it be something in the code? The data I used were the csv files from this github page and read with read.csv.

from statistics-for-data-scientists.