Why Does This Code Generate Error 91?

There are only two functions in my project: the initialization and an OK button. The initialization part does all the work. The OK button just dismisses the form. When I click the OK button, it executes the two lines you see below, then generates error 91, so I have no idea what object it is talking about. Any ideas?

Private Sub btnOk_Click()

Me.Hide

Unload Me

End Sub

Private Sub UserForm_Initialize()

Dim ssetObj As AcadSelectionSet

Dim intMode As Integer

Dim grpCode(0) As Integer

Dim dataVal(0) As Variant

Me.Hide

Set ssetObj = ThisDrawing.SelectionSets.Add("SS01")

intMode = acSelectionSetAll

grpCode(0) = 0

dataVal(0) = "KS_SHAPE"

ssetObj.Select intMode, , , grpCode, dataVal

If ssetObj.Count > 0 Then

MsgBox "Found them.", vbOKOnly

End If

ssetObj.Delete

Me.Show

End Sub