The magnetic properties of the compound were studied by magnetization measurements and neutron diffraction. The refinement of the nuclear structure shows that this compound is slightly Ni-deficient, the corresponding formula being . At 1.5 K two coexisting magnetic phases were observed in equal amounts. One of these is ferromagnetic with . The other phase is an antiferromagnetic modulated phase (antiphase domain type with two amplitudes) with . Its magnetic ordering can be described in terms of two propagation vectors ( magnetic lattice) and . The ordered magnetic moment value at 1.5 K in both phases is per Ce atom and points in the same direction along c. Above 2 K the wavevector becomes incommensurate with the crystal lattice. The observation of two magnetic phases is attributed to the occurrence of concentration fluctuations associated with the Ni deficiency.