Missing calculated field in report

[MNNovice]

I am trying to create a report that will keep remaining balance on each grant. But I am struggling with one little problem (for you gurus out there) all morning. Can someone please help?
Here is the issue at hand:

rptRunningBalance is based on qryExpDetail.
qryExpDetail is based on tblExpDetail
tblExpDetail has these fields (in addition to other): 1)ApAmount and 2)Amount
qryExpDetail has NetAmount which is calculated as:
[ApAmount]*qryGrantFunds.FedRatio

rptRunningBalance has a ProjectNo Footer which keeps this info
Budget
TotalExp = Sum([NetAmount]) + Sum([Amount])
Balance = Budget - TotalExp

Well, if a project does not have an ApAmount I get blank data for Balance. For example, if Project A has a budget of $100 and an expense in ApAmount for $10 but no amount for "Amount" I get a blank balance instead of $90.

I found out that when entering data I have disabled expense column based on category selected. For example if one selects AP then Amount column is no longer enabled. So if there is no data in Amount - my report just ignores the whole thing. When I manually entered a zero the report comes out fine. Here is the code I am referring:

Private Sub CategoryID_AfterUpdate()
    If Me.CategoryID.Value = 1 Then
        Me.ApAmount.Enabled = True
        Me.Type.Enabled = True
        Me.RefNo.Enabled = True
        Me.cboVendor.Enabled = True
        Me.txtEchoAmount.Enabled = True
        Me.Amount.Enabled = False
        Me.Type.SetFocus
 
 
    ElseIf Me.CategoryID.Value = 2 Then
        Me.ApAmount.Enabled = False
        Me.Type.Enabled = False
        Me.RefNo.Enabled = True
        Me.cboVendor.Enabled = False
        Me.txtEchoAmount.Enabled = False
        Me.Amount.Enabled = True
        Me.Date.SetFocus
 
    ElseIf Me.CategoryID.Value = 3 Then
        Me.ApAmount.Enabled = False
        Me.Type.Enabled = False
        Me.RefNo.Enabled = False
        Me.cboVendor.Enabled = False
        Me.txtEchoAmount.Enabled = False
        Me.Amount.Enabled = True
        Me.Date.SetFocus
    End If
End Sub

How can I fix this problem?


Thanks.

[ChipR]

For starters, add a field to your query defined as
Nz([Amount], 0)
and use that field for calculations instead of Amount.