Actually, code_descriptions is neither a string nor a number, it is an array type. Meaning if you want to print its string, you need to add [1] to the variable that holds the array, to get the first (and only) value in that array, or use a for each loop to iterate over all strings in that array if there can conceivably be more than one in the actual data.
tonumber on a string that does not contain a correctly formed number returns nil, which is the case in your original code; also I'm pretty sure fromJSON takes care of returning numbers correctly from a JSON where they're written as number literals rather than strings, so you shouldn't need to use tonumber unless your input data sometimes returns the number within a string.