The dark rings of the Airy pattern in the focal region of a microwave lens form phase singularities. In an error-free lens these dark rings lie on the surface of a sphere. It is shown that in the presence of primary aberrations the line singularities remain closed loops and the phase behavior near them is similar to that in the ideal lens. Third-order astigmatism of relatively small amounts produces a marked distortion of the shape of the line singularity.