access2003 #error when sum() on recalc in subform

I

idler

scene:
Access2003, on Windows 2000.

Form: main, with recordsource a table:main, which has a column:x of
type:number
with
subform:sub, with recordsource a table:sub,which has a column:x of
type:number
and text box control:xsum ,on the footer,
with controlSource: '=sum(x)' of type:number

and linkChild:x and linkMaster:x

when main is first opened, xsum displays the correct value, but when a
recalc is invoked, either by the keystroke: F9 or from VBA with me.recalc the
xsum display is: #error.

clues:
1) the #error displayed in xsum sort of flickers as if there is some compute
loop going on.
2) the #error does not happen if sub is just opened (as a regular form) and
the xsum is computed correctly and F9 / recalc can be invoked with correct
result.
3) the #error does not happen if the recalculation of xsum is the result of
moving from one sub record to another, and if the data is changed, xsum is
correctly recomputed
4) the above described #error does not occur with Access2000, where things
function correctly

I'm guessing that this is an Access2003 bug ... I've tried a variety of
things to work around but they either don't work or are really ugly.

Suggestions ?
 
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