Im using Access/Windows 2000

Im trying to use DSum() in the footer of a continuous form I want to sum the
[dblContributionAmount] field when the [dteDateCeased] field is empty to give
me a total of contributions currrently being paid. the form picks up its data
from a table called "staging_tblRegAVC"

I have the following formula in a text box in the form footer
=DSum([dblContributionAmount],"staging_tblRegAVC","[dteDateCeased] is null")

The results I get are 'strange' (for want of a better word) it seems that
access is totalling the [dblContributionAmount] of the first record multiple
times depending on whether [dteDateCeased] is null or not...Im not sure if
that makes sense but maybe the following examples will clarify....

eg#1
[Record No]    [dblContributionAmount]      [dteDateCeased]
     1                           30.00
     2                           20.00
     3                           10.00
DSum() total = 90.00

eg#2
[Record No]    [dblContributionAmount]      [dteDateCeased]
     1                           20.00
     2                           30.00
     3                           10.00
DSum() total = 60.00

eg#3
[Record No]    [dblContributionAmount]      [dteDateCeased]
     1                           10.00                         1/1/2005
     2                           30.00                         1/1/2005
     3                           20.00
DSum() total = 10.00

Thanks in advance for any help anyone can offer