Variable does not exist in the current context?

Variable does not exist in the current context?

I know this is most likely a stupid question but I am a university student
who is new to C# and object-oriented programming. I have tried to find the
answer elsewhere but I wasn't able to find anything that could help. The
debugger keeps telling me that the variable 'cust_num does not exist in
the current context'. If someone can tell me what I have done wrong and
make me feel like an idiot, I would greatly appreciate it. Thanks!
string get_cust_num()
{
bool cust_num_valid = false;
while (!cust_num_valid)
{
cust_num_valid = true;
Console.Write("Please enter customer number: ");
string cust_num = Console.ReadLine();
if (cust_num == "000000" || !Regex.IsMatch(cust_num,
@"^[0-9]+$") || cust_num.Length != 6)
{
cust_num_valid = false;
Console.WriteLine("Invalid customer number detected.
Customer numbers must be a 6 digit positive integer (zeros
will not work)");
}
}
return cust_num;
}