Question: imaginary number come out of nowhere

consider:

assume(kf1>0,kf2>kf1,q>kf1,q<kf2);

1/2*q*Pi^2*(ln(kf1)*kf1^2-ln(kf2)*kf2^2-ln(q)*kf2^2-arctanh(1/(kf1^2+q^2)^2*(kf1^4+q^4-6*q^2*kf1^2))*kf1^2+ln(q)*kf1^2-ln(2)*kf1^2+ln(-kf1+q)*kf1^2+ln(kf1^2-q^2)*q^2-ln(kf1-q)*q^2+2*kf1^2-2*q^2-1/2*ln((q-kf2)^2)*kf2^2+3*ln(kf2+q)*kf2^2+4*q*kf2-3*q^2*ln(kf2+q)+ln(2)*kf2^2+arctanh((kf2^4+q^4-6*kf2^2*q^2)/(kf2^2+q^2)^2)*kf2^2+ln(-q+kf2)*q^2-ln(kf2^2-q^2)*q^2-4*q*kf1-3*ln(q+kf1)*kf1^2+2*arctanh((-kf1^2+q^2)/(kf1^2+q^2))*kf1^2+2*arctanh((-kf2^2+q^2)/(kf2^2+q^2))*kf2^2+3*q^2*ln(q+kf1));

i.e. a real function

now we do

simplify(%)

we get:

1/2*q*Pi^2*(q^2*Pi*I+ln(kf1)*kf1^2+ln(q)*kf1^2+ln(-kf1+q)*kf1^2+ln(2)*kf2^2+arctanh((kf2^4+q^4-6*kf2^2*q^2)/(kf2^2+q^2)^2)*kf2^2+q^2*ln(-kf1+q)-ln(kf2)*kf2^2-ln(q)*kf2^2-arctanh(1/(kf1^2+q^2)^2*(kf1^4+q^4-6*q^2*kf1^2))*kf1^2-ln(2)*kf1^2-ln(kf1-q)*q^2+3*ln(kf2+q)*kf2^2-4*q^2*ln(kf2+q)-3*ln(q+kf1)*kf1^2+2*arctanh((-kf1^2+q^2)/(kf1^2+q^2))*kf1^2+2*arctanh((-kf2^2+q^2)/(kf2^2+q^2))*kf2^2+4*q^2*ln(q+kf1)-ln(-q+kf2)*kf2^2+2*kf1^2-2*q^2+4*q*kf2-4*q*kf1)

i.e. 

there is an non-diminished imaginary term...

how is this possibly be correct?