A plane problem of diffraction of acoustic waves at a thin elastic shell is considered, subject to a boundary condition defining the interaction of the shell with the incident wave. In addition to the normal derivative and the function itself, this condition contains a tangent derivative of the second order. The model problem considered is the diffraction at a circular shell. The asymptotic expansion obtained for the wave field is uniform in the insonified region and provides for continuous matching of the reflected and incident waves. The solution of this model problem clarifies the type of asymptotic expansion (ansatz) for the wave field in the problem of diffraction at an arbitrary shell.