A possible error in the semantic chapter of the ISO standard for the Z specification notation
Firstly, in addition to the Standard from 2002, there is also Technical Corrigendum 1 (TC1) from 2007 which fixes a number of issues.  I don't know of any combined document.  Both documents are available free of charge and links to them are available on the Z notation Wikipedia page.  However, for this particular question, TC1 is not relevant.
Section 15 states that &quot;only sentences of concern here are ones that are already known to be well-formed syntactically and well-typed.&quot;.  In a well-typed specification, the semantics in section 15.2.5.1 should not depend on $M$ applied outside its domain.  (Can a well-typed specification be written whose semantics depends on $M$ applied outside its domain?)  I believe this is the thrust of the comment about $\mathit{Model}$ in Table 27 that states &quot;They are applied only to names in their domains, as guaranteed by well-typedness.&quot;