In the Standard Model the hypercharges of quarks and leptons are not determined by the gauge group itself. In a recent paper [C. Hattori et al. Phys. Rev. D 83, 015009 (2011)] it is shown that, if the direct product gauge group G SM is slightly modified to the semidirect product group G ′ SM , hypercharges are restricted to quantized values as n/6 mod Z (n = 0, 1, 3, 4). In this brief paper, we examine all of the compact Lie groups locally isomorphic to G SM , and show that G ′ SM (or its isomorphisms) is the unique possibility that yields the correct hypercharge quantization.