Called object type 'int' is not a function or function pointer

#include <stdio.h>
#include <cs50.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>

char caesar_encode (char, int);  // prototype of caesar_encode function

// +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

int main (int argc, string argv[])
{
    int key = 0;
    int length, base, asciiCode;
    string sentence;
    char currentLetter, caesar_encode;

    // if wrong number of command line arguments, shout at user then quit
    if (argc != 2)
    {
        printf("Incorrect number of command line arguments.\n\n");
        return 1;
    }

    // if key parameter is too low, shout at user then quit
    key = atoi(argv[1]);
    // printf("key = %i\n\n", key);
    if (key < 1)
    {
        printf("Key parameter is too low.\n\n");
        return 1;
    }
    
    // since key values modulo 26 are equivalent for the purposes of our program
    // if a key value is larger than 26, reduce it to its modulo 26 equivalent
    // (i.e. a number from 0 to 26)
    if (key > 26)
    {
        // printf("Reducing key larger than 26 to its modulo 26 equivalent.\n");
        // printf("Old key was %i\n.", key);
        key = key % 26;
        // printf("New key is %i.\n\n", key);
        
    }
    
    // prompt user for a string
    // printf("Enter a string to encrypt: ");
    sentence = GetString();
    string newSentence = sentence; // set up newSentence same length as sentence
    length = strlen(sentence);
    
    // loop through string, adjusting each letter according to the key value
    for (int i = 0; i < length; ++i)
    {
        printf("\n");
        // get current letter from sentence
        currentLetter = (char) sentence[i];
        
        // only Caesar encrypt currentLetter is it's actually a letter
        // (as per the pset2 spec, non-alphabetic characters aren't encoded)
        if (isalpha(currentLetter))
        {
        	newSentence[i] = caesar_encode (currentLetter, key);
		}
    }
    printf("%s\n", newSentence);
    // printf("\n\n");
    return 0;
}

// +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

// caesar encoding function - accepts a char and a key value as parameters
// returns the caesar cipher encoded char
// precondition: the function is only called if it has already been determined
// that the char letter parameter is alphabetic. Therefore we don't need to test
// for this inside the function.

char caesar_encode (char letter, int key)
{
	char encoded_letter;
	int base;
	
	// CODE TO CAESAR CIPHER ENCODE THE LETTER HERE
	// if lowercase letter, base = 'a' - 1;
	// if uppercase letter, base = 'A' - 1;
    // and currentLetter = base + ((currentLetter + key) % 26)
	if (letter >= 'a' && letter <= 'z')
		base = 'a' - 1;
	if (letter >= 'A' && letter <= 'Z')
		base = 'A' - 1;
	printf("base = %i\n", base);
    
    printf("Before encoding, letter was: %c", letter);
    encoded_letter = (char) (base + (int)((int) letter + key));	// encode letter
	printf("After encoding, letter is: %c", encoded_letter);
	
	return encoded_letter;
}

// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

char caesar_encode (char, int);  // prototype of caesar_encode function

// +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

int main (int argc, string argv[])
{
    int key = 0;
    int length, base, asciiCode;
    string sentence;
    char currentLetter, caesar_encode;