We have obtained images of the planetary nebula NGC 6894 and the `stripes' near it and have analysed their morphologies. Our analysis suggests that the material of the stripes has been ionized by the central star of NGC 6894, and could have originated from the stripping of the halo of NGC 6894 by the interstellar medium (ISM). Based on the fact that the orientation of the stripes is parallel to the Galactic plane, we speculate that the morphology of the stripes is determined by the Galactic magnetic field. The above interpretation requires the motion of NGC 6894 to be in a direction which is consistent with observations. This work further stresses the significant role of the ISM magnetic field in shaping haloes of planetary nebulae, and the potential for using planetary nebulae to probe the ISM.