add new Sketch in DerivedPartComponents

[ssyXENBT]

Hey, 

I need to insert a sketch in a Derived Part.

after selecting a face in an assembly, I query whether the part has a Derived Part.
Afterwards I look for the same face in the Derived Part and save it, which is then passed to a function.
Then I use the face for the add sketch command, but always get the error : HRESULT: 0x80070057 (E_INVALIDARG)

Any Ideas? 

Sub: 

    Private Sub btn_comp_Click(sender As Object, e As EventArgs) Handles btn_comp.Click
        Dim oPartFaceSelection
        Dim SketchComp As ComponentOccurrenceProxy
        Dim funcBool As Boolean
        Dim oPartFace1Selection As Edge

        If language = "de-DE" Then
            oPartFaceSelection = ivExe.CommandManager.Pick(SelectionFilterEnum.kPartFaceFilter, "Skizziereben auswählen")

        ElseIf language = "fr-FR" Then
            oPartFaceSelection = ivExe.CommandManager.Pick(SelectionFilterEnum.kPartBodyFilter, "Sélectionner le plan d'esquisse")
        End If

        Dim oPartOcc As ComponentOccurrence = oPartFaceSelection.ContainingOccurrence
        Dim oPartSelection

        If oPartOcc.Definition.ReferenceComponents.DerivedPartComponents.Count >= 1 Then
            Me.lbl_comp.Text = oPartOcc.Name & " (" & oPartOcc.Definition.ReferenceComponents.DerivedPartComponents.Item(1).Name & ")"

            oPartSelection = oPartOcc.Definition.ReferenceComponents.DerivedPartComponents.Item(1)
            Dim pryBody As ReferenceFeature = oPartSelection.PrimaryBody
            Dim bodyFaces As Faces = pryBody.Faces

            For x = 1 To bodyFaces.Count - 1
                Dim face As Face = bodyFaces.Item(x)
                If face.InternalName = oPartFaceSelection.InternalName Then
                    oPartFaceSelection = bodyFaces.Item(x)
                End If
            Next

            funcBool = True
        Else
            Me.lbl_comp.Text = oPartOcc.Name
            oPartSelection = oPartOcc
            funcBool = False
        End If
        Console.WriteLine(TypeName(oPartFaceSelection))
        createSketch(oPartFaceSelection, oPartSelection, funcBool)

    End Sub

Function: (Error on Line16)

Function createSketch(Sketchface, oPartDoc, bool)
        Dim oSketch As PlanarSketch
        Dim oExistisSketches
        Dim sketchCounter As Integer
        sketchName = "Sketch_" & sketchCounter

        If bool = True Then 'dervied Parts
            oExistisSketches = oPartDoc.Definition.Sketches
            For s = 1 To oExistisSketches.Count
                If oExistisSketches.Item(s).ReferencedEntity.Name = sketchName Then
                    sketchCounter += 1
                    sketchName = "Sketch_" & sketchCounter
                End If
            Next
            If Sketchface.SurfaceType = SurfaceTypeEnum.kPlaneSurface Then
                oSketch = oPartDoc.ReferencedDocumentDescriptor.ReferencedDocument.ComponentDefinition.Sketches.Add(Sketchface)
                oSketch.Name = sketchName
                'Dim docOpen = ivExe.Documents.Open(oPartDoc.ReferencedDocumentDescriptor.FullDocumentName)
            End If
        Else 'No dervied Parts                   
            oExistisSketches = oPartDoc.Definition.Document.ComponentDefinition.Sketches
            For s = 1 To oExistisSketches.Count
                If oExistisSketches.Item(s).Name = sketchName Then
                    sketchCounter += 1
                    sketchName = "Sketch_" & sketchCounter
                End If
            Next
            If Sketchface.SurfaceType = SurfaceTypeEnum.kPlaneSurface Then
                oSketch = oPartDoc.Definition.Document.ComponentDefinition.Sketches.Add(Sketchface.NativeObject)
                oSketch.Name = sketchName
            End If
        End If
    End Function

Thank you!