ungültiger Verweis auf Next-Steuervariable

Private Sub ComboBox1_Click()
Dim i As Integer
Dim a As Integer
If ComboBox1.ListIndex = 0 Then Exit Sub
TextBox1 = Sheets("Merker").Cells(ComboBox1.ListIndex + 1, 1)
For i = 2 To 22
For a = 1 To 21
If Sheets("Merker").Cells(ComboBox1.ListIndex + 1, i) = "ja" Then
Controls("CheckBox" & a).Value = True
End If
Next i
Next a
End Sub