Freeform dcl-ds gives compile error when fixed form DS does not

On midrange-l I just got some much needed SQL help that I greatly
appreciate. I've got the SQL Insert working correctly in my RPG program
and am now working on reading that file joined to others via a cursor.

I'm wanting to define some external data structures so fields are defined.

If I put this in the source code:

D cusrbtmstDS E DS ExtName(CUSRBTMST)
D Inz

it compiles just fine. But if I put this in:

Dcl-ds cusrbtmstDS ExtName(CUSRBTMST) Inz;

I get this compile error:

SQL0314 Position 28 Host variable WCUSNR not unique.

on this statement:

Exec SQL Fetch Next
from RbtCursor
Into :wCUSNR,
:wCUSRBTPCT,
:wCTCODE,
:wSLSYYYYMM,
:wSLSAMT,
:wCRDAMT,
:wCUSNM,
:wCTDESC,
:wCTGLAC,
:wCTGLSA;

WCUSNR is defined like this:

D wCUSNR S 4S 0

wCUSNR is not defined anywhere else.

Why can I define the DS in fixed format, but not freeform? What am I
missing?

If it's beneficial, this is where I declare the cursor:

Exec SQL Declare RbtCursor cursor
for Select rbt.CUSNR,
pct.CUSRBTPCT,
rbt.CTCODE,
rbt.SLSYYYYMM,
rbt.SLSAMT,
rbt.CRDAMT,
cus.CUSNM,
cat.CTDESC,
cat.CTGLAC,
cat.CTGLSA
From CUSRBTSLS rbt
Left Outer Join
CUSRBTMST pct On rbt.CUSNR = pct.CUSNR
Left Outer Join
DMCUSMST cus On rbt.CUSNR = cus.CUSNR
Left Outer Join
DMCATMST cat On rbt.CTCODE = cat.CTCODE
Order By 1, 2;