Rectifying visual basic code

There is a field called "Contract ID"
Contract ID is a unique value to each record.
But for my new records that are entered Contract ID field is no more used instead a field  "FO No" is used.
Each Contract ID/FO No can have many vendors that is a one to many relationship.
For that a subform was created in the main form to view the vendors for each contract id/fo no.
Given below is the code I have for linking the subform and and main form with the
contract id/fo no.

The problem is that it only works when the contract id/fo no is a number like 12345
but when it is anything like abc123 then it shows me "Type Mismatch"
Both the fields in both my tables are in "Text" form.

I think it is something to do with:-
Dim lngID As String

Please help

Private Sub Command281_Click()
On Error GoTo Err_Command281_Click
 
   Dim strWhere As String
Dim lngID As String
 
If "" & Me.Contract_ID = "" Then
       strWhere = "[FO No]=" & Me.FO_No
       lngID = Me.FO_No
 
Else
      strWhere = "[Contract ID]=" & Me.Contract_ID
      lngID = Me.Contract_ID
 
End If
 
DoCmd.OpenForm "Job Request1", , , strWhere
If "" & lngID = Me.FO_No Then
 
Forms![Job Request1].[FO No].DefaultValue = lngID
Else
Forms![Job Request1].[Contract ID].DefaultValue = lngID
 
 
    End If
 
Exit_Command281_Click:
    Exit Sub
 
Err_Command281_Click:
    MsgBox Err.Description
    Resume Exit_Command281_Click
    
End Sub

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