[Python] Cannot cast array data from dtype('complex128') to dtype('float64') according to the...

I am trying to solve an optimization problem as shown below. But every time I get an error Cannot cast array data from dtype('complex128') to dtype('float64') according to the rule 'safe'. Can anyone help me to see what is wrong here in the code?

def func(vec):

linspec = -(kx**2)+((1.-nu)*(kx**4))
lin = linspec*np.fft.fft(vec)
nlin = np.zeros_like(lin)
nlinre = vec*vec
nlinspec = np.fft.fft(nlinre)
nlin = (0.5*1j*kx*nlinspec)
sol = lin+nlin
rhs = np.zeros_like(sol, dtype='complex')
sol -= rhs
sol = np.fft.ifft(sol).real

return sol

def kssol(u0):

u1 = np.ones((2*Mx,), dtype='complex')
#u1 = 100.*u0
u = scipy.optimize.fsolve(func, u1)

return u

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