We have measured the thermal expansion of Ni nanowires (NWs) electrodeposited into self-organized nanoporous amorphous aluminum oxide (AAO) membranes without an Al substrate using X-ray diffraction between 110K and 350 K. Assuming a bulk-like thermal expansion of the isolated Ni NWs, this result indicates that AAO also has a negative thermal expansion. We estimate the thermal expansion of nanoporous AAO . We show that data obtained previously on the thermal expansion of metallic NWs grown in the nanoporous AAO may be interpreted as originating from a negative thermal expansion of the matrix.