Validator shows an Error on a SHOULD constraint

[dan venton]

National Health Care Survey -- Errors:
Message If no code is available use nullFlavor="OTH" and enter the value as free text in code/originalText. SHALL contain exactly one [1..1] value with @xsi:type="CD", where the code SHOULD be selected from CodeSystem ICD10 (2.16.840.1.113883.6.3) (CONF:1106-636).
Context /ClinicalDocument[1]/component[1]/structuredBody[1]/component[6]/section[1]/entry[6]/observation[1]/entryRelationship[1]/observation[1]
Test count(cda:value[@codeSystem='2.16.840.1.113883.6.3' or @nullFlavor][@xsi:type='CD'])=1

Even though the IG says,

SHALL contain exactly one [1..1] value with @xsi:type="CD", where the code SHOULD be
selected from CodeSystem ICD10 (urn:oid:2.16.840.1.113883.6.3) (CONF:1106-636)

AND

Value MAY be selected from ICD-9/10/CM (based on the current version in US
realm) (CONF:1106-860).

Additionally 6.3 is ICD-10 International, which is a weird choice of which ICD-10 to use.