In Luau, the argument type information for string.char is wrong; it has the first argument type as string, which is incorrect.
As an example, enter string.char(1) in a script. Even outside of strict mode, it will emit an error saying This function must be called with self. Did you mean to use a colon instead of a dot?
Replacing the 1 with "1" will cause that error to go away (though it will still complain until a second argument is provided).
As of May 20th the type signature for string.char was: (string,number,number?,number?,number?)->(string)
It should instead be: (number) -> (string) or something to that effect. string.char accepts a vararg so maybe it should be () -> string.
This is occurring in Studio version 0.442.3.409454.