  We present the results of a study of the vortex lattice (VL) of the nickel
chalcogenide superconductor TlNi2Se2, using small angle neutron scattering.
This superconductor has the same crystal symmetry as the iron arsenide
materials. Previous work points to it being a two-gap superconductor, with an
unknown pairing mechanism. No structural transitions in the vortex lattice are
seen in the phase diagram, arguing against d-wave gap symmetry. Empirical fits
of the temperature-dependence of the form factor and penetration depth rule out
a simple s-wave model, supporting the presence of nodes in the gap function.
The variation of the VL opening angle with field is consistent with earlier
reports of of multiple gaps.
