We say 

[Definition: If a parameter has an implicit default value which cannot be converted to the required type (that is, if it has an as attribute which does not permit the empty sequence), then the parameter is implicitly mandatory.]

However, the implicit default may be either the zero-length string or the empty sequence. So the parenthetical remark in this definition should more accurately say

(that is, if it has an as attribute which does not permit the zero length string or the empty sequence as appropriate)