Bad Argument Errors [?] Click For Detail.

Hello.

I got this error in a few lines;

local String = CB.Text.Value 
local Length = string.len(String)

This is only Part of my script.

CB is a text label. Text is a string value.

This is what the error said;

string expected got nil. near 'len'

If you know a way to fix it then please help with examples.