not all codepaths return a value

Error : "not all code path return a value"

what's the problem in this program..the program is to find the given number is a palindrome or not

class Program

{

static void Main(string[] args)

{

int number;

number = Convert.ToInt32(Console.ReadLine());

int rev = rev1(number);

Console.WriteLine(rev);

if (rev == number)

Console.WriteLine("is a palandrome");

else

Console.WriteLine("not a palandrome");

Console.ReadLine();

}

private static int rev1(int number)

{

int sum = 0;

if(number==0)

{

int j = number % 10;

sum = sum * 10 + j;

int c = sum;

rev1(number / 10);

}