<xs:element name="spare" dfdl:length="{ $tns:WordPaddingBits }">
  <xs:simpleType dfdl:lengthKind="explicit">
   <xs:annotation>
    <xs:appinfo source="http://www.ogf.org/dfdl/">
      <dfdl:assert test="{ . eq 0 }" />
    </xs:appinfo>
   </xs:annotation>
   <xs:restriction base="xs:unsignedInt">
    <xs:enumeration value="0" />
   </xs:restriction>
  </xs:simpleType>
</xs:element>

Now, suppose the variable tns:WordPaddingBits has value 0. 

So this padding integer will be length 0 (this binary data with lengthUnits bits). 

Currently, Daffodil gives a runtime SDE on this. The assert test expression complains that "." has no value.
I would claim this should either provide a value of 0 for this integer, or it should be a parse error because you must have at least 1 bit.

Thoughts?