The v3 concept map for AdministrativeGender (http://hl7.org/fhir/cm-administrative-gender-v3.html) includes a mapping from `unknown` to `UNK`, but `UNK` does not appear to be part of the destination value set (http://hl7.org/fhir/v3/AdministrativeGender/vs.html). According to Lloyd McKenzie, "Ideally our build process should be checking that the codes listed in the ConceptMaps actually exist in the expansions of the source and target value sets, which would have caught this issue."