Strange problem with double integrals

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I need to calculate a double of integral a large expression numerically. But, I found a strange error. The code:

def doubleIntegral():
    def f2(theta):
        def f1(phi):
            return theta*phi/pi
        return numerical_integral(f1,0,2*pi)[0]
    return numerical_integral(f2,0,pi)

works. but

a = theta*phi/pi  # Suppose this is a huge expression that came out of previous calculations

def doubleIntegral():
    def f2(theta):
        def f1(phi):
            return a
        return numerical_integral(f1,0,2*pi)[0]
    return numerical_integral(f2,0,pi)

doesn't work

unable to simplify to float approximation
unable to simplify to float approximation
.
.
.

What is going on? And how can I fix this?