input: Write a function to debug the given Python code where two functions `f1` and `f2` are defined. The function `f1` should take an input `x` and return `x + 1`, and the function `f2` should call `f1` twice with the same input `x` and return the product of the two results. The function should then print the result of calling `f2` with the input `2`.
  
  def debug_code():
      def f1(x):
          return x + 1
  
      def f2(x):
          return f1(x) * f1(x)
  
      print(f2(2))
  
  assert debug_code() == None
output: def debug_code():
      def f1(x):
          return x + 1
  
      def f2(x):
          return f1(x) * f1(x)
  
      print(f2(2))