Question: Coeff dosn't work correctly

Hi

coeff dosn't work correctly in maple 17

Here is the code

restart;

E := m1*(diff(x1(t), t, t))+3*k*x1(t)-k*x3(t)-k*x2(t);
coeff(E, x1(t));
Error, unable

 

the coeff of diff(x1(t),t,t) and x2(t) and x3(t) can be determined and are true, but the coeff of x1(t) can not be determined??!!!!!!!!!!!!

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