'negative number to a fractional power not real' while computing inegral

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Hi

I want to compute the following integral:

B=40
Ef=1.13*10^(-18)
KT=4.03*10^(-21)
mu=1.732*9.27*10^(-24)
f(t)=(sin(t)*(cos(t))^(3/2))/(exp((B*mu*cos(t)-Ef)/KT)+1)
numerical_integral(f,0,pi)

However, it gives me an error

"Exception ValueError: ValueError('negative number to a fractional power
not real',) in 'sage.gsl.integration.c_ff' ignored"

Can somebody resolve this?

Thanks and regards Ajit Kumar