TypeError for derivative function

I used sympy to find derivative of a function.

𝑓(𝑥)=𝑥−5+√(4−𝑥^2)

import sympy
def f_derivative(x):
  x = sympy.Symbol('x')
  f = x - 5 + (4 - x**2)**0.5
  derivative_f = f.diff(x)
  derivative_f = sympy.lambdify(x, derivative_f)
  print(derivative_f(1))
f_derivative(1)
assert f_derivative(1) - 0.42264973 < 1e-5

However, there is an error when I use assert to check

TypeError: unsupported operand type(s) for -: ‘NoneType’ and ‘float’

Please let me know how to fix it

>Solution :

import sympy
def f_derivative(x):
  x = sympy.Symbol('x')
  f = x - 5 + (4 - x**2)**0.5
  derivative_f = f.diff(x)
  derivative_f = sympy.lambdify(x, derivative_f)
  result = derivative_f(1)
  assert result - 0.42264973 < 1e-5
  print(result)
f_derivative(1)

I hope helped you