Using %SIZE() on database fields in D-Spec

I am attempting to use the %size() function on a d-spec to define a field in
ILE/RPG. The field in the %size() function is a field that is coming from a
program file. The compiler seems to think that the field does not exist.
However, if I add an externally described data structure to bring in the
field definitions before the %size() function is used everything works fine.
What gives?

This does not work

F file1  IF ....
D ds1    DS  
D   fld1         overlay(dbfld1:%size(dbfld1))  //fld1 overlays last byte of
dbfld1
C .....

However, this works fine.

F file1  IF ....
D ds0  E DS     extname(file1)
D ds1    DS
D   fld1        overlay(dbfld1:%size(dbfld1))

I know the database fields are brought into the program until the I specs
are processed for externally described files, but why would that matter in
this age of multi-pass compilers.

-Walden
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