spectral factorization of improper systems

My W1 and W2 are

W1=zpk((1+0.1*s)/(s+0.1));
W2=zpk(0.1*(s+0.1));

I want to find spectral factorization of using spectralfact command

spectralfact(W1*W1'+W2*W2)

but it is giving the error

Error using DynamicSystem/spectralfact (line 58)

Not supported for improper (non causal) continuous-time models.

Although the spectral factorization is posible for W1*W1'+W2*W2