We study the core mass function (CMF) of the massive protocluster G286.21+0.17 with the Atacama Large Millimeter/submillimeter Array via 1.3. mm continuum emission at a resolution of 1 ''.0 (2500 au). We have mapped a field of 5 3. x. 5'.3 centered on the protocluster clump. We measure the CMF in the central region, exploring various core detection algorithms, which give source numbers ranging from 60 to 125, depending on parameter selection. We estimate completeness corrections due to imperfect flux recovery and core identification via artificial core insertion experiments. For masses M greater than or similar to 1 M-circle dot, the fiducial dendrogram-identified CMF can be fit with a power law of the form dN/dlog. M proportional to M-alpha with alpha similar or equal to 1.24. +/-. 0.17, slightly shallower than, but still consistent with, the index of the Salpeter stellar initial mass function of 1.35. Clumpfind-identified CMFs are significantly shallower with alpha similar or equal to 0.64. +/-. 0.13. While raw CMFs show a peak near 1 M-circle dot, completeness-corrected CMFs are consistent with a single power law extending down to similar to 0.5 M-circle dot, with only a tentative indication of a shallowing of the slope around similar to 1 M-circle dot. We discuss the implications of these results for star and star cluster formation theories.