TypeError 'unable to simplify to float approximation' while trying to define an integral operator

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Hello, I am trying to define an integral operator acting on a subspace of $L^2(\mathbb{R})$ that depends of time using sage. Being more explicit $W(t)$ takes a real function $f$ and returns another function $g$ defined by $$\begin{equation} W(t)f(x)=g(x)=\int_{-\infty}^{+\infty}K(x,y,z)f(y)\text{d}y \end{equation}$$ This is the code I came up with

def W(t):
def dummy2(f):
    def dummy3(x):
        integrand(y)=f(y)*K(x,y,t)
        return numerical_integral(integrand,-Infinity,+Infinity,algorithm='qag')[0]
    return dummy3 
return dummy2

With $K$ a reasonable function of $x,y,t$ However I am getting the following error

TypeError: unable to simplify to float approximation

I am pretty sure it's related to the types passed to numerical_integrand What could be a solution? Or a better way to implement it? Thank you all