VBA Access Create Calculated Field

I am trying to create a calculated field in access using VBA. I have other code working for regular field types (Text,Number,etc). When it comes to the calculated field, the field is not added to the access table. I think it has something to do with the expression.

Code:

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Set db = CurrentDb()
...
'This code works fine and the fields are added to access table
For Each strColumn In strColumns

Set columnName = xlsht.Cells(1, strColumn)

If columnName = "National ID" Then
  Set columnName = xlsht.Cells(1, "SSN")
End If

If columnName <> "" Then
  Set FieldName = tb1.CreateField(columnName, dbText, 150)
  tb1.Fields.Append FieldName
End If
 
Next strColumn
...

'When I try to add calculated Field, it is not added to table
fd1 = "Decease?"

Set f1 = tb1.CreateField(fd1)
f1.Expression = IIf(IsNull([Dt of Death]), 0, IIf([covg elect] = "T", 0, 1))
tb1.Fields.Append f1

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