I simply can't find this logic error.

[totalanonymity]

My professor said to find the logical error within the char shift function, but I can't figure it out. This course is accelerated and so is moving a bit more quickly than I can keep up at times!

Here is the code:

#include <iostream>
#include <string>
using namespace std;

const string empty_string = "";

char shift(int shift_amount, char original);
string shift(int shift_amount, const string& original);

int main()
{
	string original = "Of all things, never to have been born is best!";
	for(int shift_amount = 0; shift_amount <= 26; shift_amount++)
	{
		string shifted = shift(shift_amount, original);
		cout << shifted << endl;
	}

	system("pause");
	return 0;
}
char shift(int shift_amount, char original)
{
	char shifted;
		
	static const int ALPHA_SIZE = 26;
	static const int upper_base_code = static_cast<int>('A');
	static const int lower_base_code = static_cast<int>('a');

	shift_amount = shift_amount % ALPHA_SIZE;
	if(shift_amount < 0)
		shift_amount += ALPHA_SIZE;	

	int base_code;
	if(isupper(original))
		base_code = upper_base_code;
	else
		base_code = lower_base_code;

	int original_code = static_cast<int>(original);
	int original_offset = original_code - base_code;
	int shifted_offset = (original_offset + shift_amount) % ALPHA_SIZE;
	int shifted_code = shifted_offset +  base_code;
	shifted = static_cast<char>(shifted_code);
	return shifted;
}
string shift(int shift_amount, const string& original)
{
	string shifted = empty_string;
	int length = original.length();
	for(int index = 0; index < length; index++)
		shifted += shift(shift_amount, original[index]);
	return shifted;
}

Any help would be appreciated.

Note: I would actually prefer that tips and hints be provided, as well as clarification to understand this matter better instead of a straight out answer. Granted, if none of the above can be satisfied, helping directly with the answer wouldn't prove terrible.