Type Mismatch Error When Loop Through the Field Names in a Query

I am getting a type mismatch error when attempting to loop through the fields in a recordset:

Public Function fnCompareTableHeaders(strFirstQuery As String, strSecondQuery As String) As Boolean
On Error GoTo ErrHandler

    Dim db As Database
    Dim rstFirstTable As dao.Recordset
    Dim rstSecondTable As dao.Recordset
    Dim strFirstString As String
    Dim strSecondString As String
    Dim fld As Field

    DoCmd.Hourglass True
    DoCmd.SetWarnings False
    
    Set db = CurrentDb
    'create recordsets for each table table to be compared
    Set rstFirstTable = db.OpenRecordset(strFirstQuery, dbOpenDynaset)
    Set rstSecondTable = db.OpenRecordset(strSecondQuery, dbOpenDynaset)
    
    'loop through first table and write field names to string
    For Each fld In rstFirstTable.Fields
        If strFirstString = "" Then
            strFirstString = fld.Name
        Else
            strFirstString = strFirstString & "|" & fld.Name
        End If
    Next
    
    'loop through second table and write field names to string
    For Each fld In rstSecondTable.Fields
        If strSecondString = "" Then
            strSecondString = fld.Name
        Else
            strSecondString = strSecondString & "|" & fld.Name
        End If
    Next
    
    'compare field name strings to determine same fields in each table
    If strFirstString = strSecondString Then
        fnCompareTableHeaders = True
    Else
        fnCompareTableHeaders = False
    End If
    
    
ExitFunct:
  DoCmd.Hourglass False
  DoCmd.SetWarnings True
  Set rstFirstTable = Nothing
  Set rstSecondTable = Nothing
  db.Close
  Exit Function
      
ErrHandler:
    MsgBox Err.Description, vbExclamation, Err.Number & " - Error Message"
    Err.Clear
    fnCompareTableHeaders = False
    Resume ExitFunct

End Function

The error specifically says 'Type Mismatch'. The variables passed to the function are two queries, both with the same data (in theory). This function is the start of validating is that they do indeed match by verifying with both tables have the same header names.

The error is specifically thrown at

For Each fld In rstFirstTable.Fields
        If strFirstString = "" Then
            strFirstString = fld.Name
        Else
            strFirstString = strFirstString & "|" & fld.Name
        End If
    Next