Error in Laplace transform with sympy: "Argument 'polar_lift(0)' is not comparable."

Question

I've been trying to calculate the Laplace transform of the following function:

x, u, k = symbols('x u k', real = True)

def gLbar(x):
   return Piecewise((0, x < 0), (k*exp(-k*x), (x >= 0)&(x <= u)), (0, True))
gLbar(x)

Which is basically an exponential distribution of parameter k that is truncated at u. u and k are scalars whose values I don't want to define just yet (so far that has worked for integration/derivation operations).

I try to compute the Laplace transform with the sympy function, which has worked so far for many other functions:

laplace_transform(gLbar(t), t, s)

However, for this one, I get an error:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-149-5e61d42f0daf> in <module>()
----> 1 laplace_transform(gLbar(t), t, s)


/usr/local/lib/python3.7/dist-packages/sympy/functions/elementary/miscellaneous.py in _new_args_filter(cls, arg_sequence)
    559                     arg.is_number and
    560                     not arg.is_comparable):
--> 561                 raise ValueError("The argument '%s' is not comparable." % arg)
    562 
    563             if arg == cls.zero:

ValueError: The argument 'polar_lift(0)' is not comparable.

Do you know what might be the problem? I'm new to using sympy but so far everything was working OK.

Thank you!!