Error: The evaluation of the expression is problematic.

[Student Stuttgart]

Consider following MWE:

import sympy as sp
import sympy.plotting.plot as symplt

s = sp.symbols('s')
t = sp.symbols('t', positive = True)
Xs = (s + 101) / (s**2 + 101 * s + 3)
xt = sp.inverse_laplace_transform(Xs, s, t)
symplt(xt, (t, 0, 20))

I obtain following error:

python3.6/site-packages/sympy/plotting/experimental_lambdify.py:230: UserWarning: The evaluation of the expression is problematic. We are trying a failback method that may still work. Please report this as a bug.
  warnings.warn('The evaluation of the expression is'