When field workers go out to measure people from their feet they will inevitably encounter someone who does not have feet. The ambiguity of how the definition applies to this case can easily lead to field workers doing weird things like substituting (non-randomly) people with feet for those without, or trying to guess "if this person had feet" or simply omitting that unit from the sample entirely.

Either one of those methods are fine, but leaving it unspecified is a threat to external validity.