Question: dsolve. Error, (in ODEtools/odsolve) invalid input: `PDEtools/sdsolve`

Hi,

Really, can't understand why there's a problem in the line desolve

restart;

eq1 := m*(diff(x1(t), t, t))-f = 0;

f := -k*x1(t)+k*(x2(t)-x1(t))-lambda*(diff(x1(t), t)) ;

g := -k*(x2(t)-x1(t)); eq2 := m*(diff(x2(t), t, t))-g = 0;

fcns := {x1(t), x2(t)};

ICS := {x1(0) = 1, x2(0) = 0, D(x1)(0) = 0, D(x2)(0) = 0};

sys := {eq1, eq2}; m := 1; k := 1; lambda := 1; sysdiff := sys union ICS;

Sol := desolve({sysdiff}, {fcns});

Error, (in ODEtools/odsolve) invalid input: `PDEtools/sdsolve` expects its 1st argument, SYS, to be of type {set({`<>`, `=`, algebraic}), list({`<>`, `=`, algebraic})}, but received {{diff(diff(x2(t), t), t)+x2(t)-x1(t) = 0, diff(diff(x1(t), t), t)+2*x1(t)-x2(t)+diff(x1(t), t) = 0, x1(0) = 1, x2(0) = 0, (D(x1))(0) = 0, (D(x2))(0) = 0}}

???

have u got an idea ?