Getting an error while using 'isdigit();'

[Guillermoso]

Hello, im getting an 'exception: std::out_of_range at memory location 0x00AFF71C.' error when program gets to the 'isdigit' function. I dont really see why, sending a char to evaluate, expecting a true or false in return. heres my code. (Just included relevant code. Ill be checking for answers regualrly, if any more code is nedded ill post it in the comments.)

      string expresion, posfija;
	char x;
	Pila<char> s;

	header();//just a line of text
	cout << "\t Expresion to convert." << endl;
	cin >> expresion;
	cin.ignore();
	system("cls");
			do
			{
				x = expresion.front();
				expresion.erase(expresion.front());

				if (isdigit(x))
	         		{
					posfija.push_back(x);
				}
   //the do-while continues for a while, but i get the error right at the is digit function. 

[ne555]

http://www.cplusplus.com/reference/string/string/erase/
you are calling this version string& erase (size_t pos = 0, size_t len = npos);

pos
Position of the first character to be erased.
If this is greater than the string length, it throws out_of_range.
Note: The first character in str is denoted by a value of 0 (not 1).