CS unreachable from current segment

When defining a code label using colon (:), LABEL or PROC, the CS register
is not assumed to either the current code segment or to a group that
contains the current code segment. For example,

   PROG1 SEGMENT
      ASSUME cs:PROG2 
   START:                ;error, bad CS assume 

This error only occurs in MASM mode. Ideal mode correctly handles this
situation.