On my computer:

str_sub("TKQTAR", 2, 2) <- "Kme1"; x

returns

Error in str_sub(“TKQTAR”, 2, 2) <- “Kme1” :
target of assignment expands to non-language object

while if I write the same code passing the string as an argument it works.

x <- "TKQTAR"
str_sub(x, 2, 2) <- "Kme1"; x

returns “TKme1QTAR”

why is that?