VBA Data Validation populate error in For Next loop

[Rowland Hamilton]

Can anyone explain why this code breaks on row 21,833 after working since row 8? Note: vntMatch is defined as variant and does not change. UsedRange is getting the proper row count (27,852)
Dim vntMatch As Variant

 vntMatch = Application.Match("Manual", shtCashForecast.Range("7:7"), 0)
   If Not IsError(vntMatch) Then
        'Loop through Manual column, offsets to start with cell under header.
        For Each rCell In shtCashForecast.Range("7:7").Cells(1, vntMatch).Offset(1, 0).Resize(shtCashForecast.UsedRange.Rows.Count, 1).Cells
            
            'Adds validation to Manual column cells if total forecast for row > 0
            'Changing offset from -1 to -3 since added 2 columns
            If InStr(rCell.Offset(0, -3).Formula, "=") > 0 Then
                With rCell.Validation
                    .Delete
                    .Add Type:=xlValidateList, AlertStyle:=xlValidAlertStop, Operator:= _
                    xlBetween, Formula1:="=ManualMonths"
                    .IgnoreBlank = True
                    .InCellDropdown = True
                    .InputTitle = ""
                    .ErrorTitle = ""
                    .InputMessage = ""
                    .ErrorMessage = ""
                    .ShowInput = True
                    .ShowError = True
                End With
            End If
        Next
   End If

 

[Rowland Hamilton]

Breaks at Instr function I believe

 

[Rowland Hamilton]

ManualMonths=Controls!$A$2:$A$8 and is a named range with a workbook wide scope. It is constant through the loop

 

[Rowland Hamilton]

Correction: Doesn't break on Instr, but rather on

                    .Add Type:=xlValidateList, AlertStyle:=xlValidAlertStop, Operator:= _
                    xlBetween, Formula1:="=ManualMonths"