Sympy: simplify gives TypeError: cannot determine truth value of Relational

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I have a complicated sympy expression and I want to simplify it. However, I get a TypeError. The following is a shortened version of my expression but reproduces the error:

from sympy import *
x = Symbol('x')
y = Symbol('y')
ux = Function('ux')(x,y)
uy = Function('uy')(x,y)
a = ((Derivative(ux, x)*Derivative(uy, y)+ 1.0*Derivative(ux, x) - Derivative(ux, y)*Derivative(uy, x) + 1.0*Derivative(uy, y) + 1.0)
     *(1.0*Derivative(ux, x)*Derivative(uy, y) + 1.0*Derivative(ux, x) - 1.0*Derivative(ux, y)*Derivative(uy, x) + 1.0*Derivative(uy, y) + 1.0)) / (Derivative(uy, y) + 1.0)
b = simplify(a)

Can anybody explain what is the problem?

Remark: The multiplication with 1.0 can be removed (and then it works without any error), however the expression is automatically created and at the moment I dont see how to do this removal.

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