Magnetic field induced Freedericksz transition in the twist geometry of an initially well-aligned nematic solution of a rod-like polymer, polyhexylisocyanate (PHIC), is studied. For the angle a between the magnetic field and the unperturbed director no, one finds a homogeneous distortion if this angle is less than 45 degrees. For angles greater than 45 degrees, the reorientation is inhomogeneous, and is coupled to secondary flow, giving rise to spatially periodic structures. The director deformation involves an out-of-plane tilt at short times, which manifests itself as a phase grating for transmitted polarized light. It is found that the wavelength of the instability is independent of the magnetic field strength in the experimental range, contrary to prior results for nematic solutions of polybenzylglutamate. We postulate that the insensitivity of the wavelength of the periodic structure is due to the nonflow-aligning character of PHIC solutions.