Any genius??out there that can help me with this one?

I'm using Access 2002 on WinXP.

What I have is a document ID number system. The ID can be formatted in the following ways:

000.000
000.000.00
000.000.0000

What I?? running into is when the DLookup finds a match that is greater than 7 characters I get a type mismatch error. I thought that it may be looking at it as a number, so I??e added a line of code that converts it back into a string if that was the case. I still get the error either way. Here?? the code:

Private Sub cmbCatNo_AfterUpdate()
On Error GoTo Err_cmbCatNo_AfterUpdate

Dim strCatNo As String
Dim strSecNo As String
Dim strSeqNo As String
Dim dblConvertNo As Double

'Three digit category number pulled from a control on the form.
strCatNo = [Forms]![frmDocumentNamingForm]![cmbCatNo]
'First number in sequence for the secondary number control.
strSecNo = ".001"
'First concantinated sequence number to compare.
strSeqNo = "" & strCatNo & strSecNo & ""

'Clears secondary number field on form.
[Forms]![frmDocumentNamingForm]![txtSecNo] = ""

'Looks for a match in database to the latest sequenced number. FileID is a string datatype.
Do While DLookup("[FileID]", "DocumentMaster", "[FileID] Like '" & strSeqNo & "*'")

'Converts the string into a decimal and add the next sequenced number.
dblConvertNo = CDbl(strSeqNo) + 0.001
'Reformats the number for comparison.
strSeqNo = Format(dblConvertNo, "000.000")
'Converts the data back into a text string.
strSeqNo = "" & CStr(strSeqNo) & ""
MsgBox strSeqNo

Loop

'If no match is found, it inputs the right three digits of the sequence number into the secondary number control.
[Forms]![frmDocumentNamingForm]![txtSecNo] = Right(strSeqNo, 3)

Exit_cmbCatNo_AfterUpdate:
Exit Sub

Err_cmbCatNo_AfterUpdate:
MsgBox Error$
Resume Exit_cmbCatNo_AfterUpdate

End Sub

Hi,

The DLookup function return a field value in a domain, or specified set of records.

it takes the following parameters.

The syntax of the DLookup() function is as follows:

   DLookup(Expression, Domain [, Criteria])

The criteria argument is an optional string expression that you can use 

to restrict the range of the data that the DLookup() function is performed on.

Note that the criteria argument is identical to the WHERE clause in an SQL 

expression (except that you do not use the keyword WHERE).

You can Specify a Numeric Criteria, Numeric Criteria that Comes 

from a Field on a Form and Textual Criteria.

Please refer to the link below for additional information and trouble shooting

http://support.microsoft.com/?kbid=208786#XSLTH3120121123120121120120

For further information on the criteria that can be used please refer below:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vbaac11/html/achowSpecifyTextCriteriaControlForm_HV05188171.asp